lib/strconv: f64tos Ryū shortest float→string (#106 fold-5a)

Graduate f64tos from the lossy fixed-point placeholder to Ryū shortest-
round-trippable (ftos.ha:432 + ftos_ryu.ha). f64tos(n:f64) str, G-format
(void/NONE) — the faithful documented subset (full parametric fftosf is
dead code for G/void/NONE → deferred #64). Ryū core decomposed: struct-
RETURN + scalar params (Hare's r128 idiom; avoids tuple-ABI #163-166).
f64 powers tables [15][2]/[13][2]u64 (2D #156). Zero float literals.
Both E+F encode paths reachable+tested, no dead code.

Graduation (lib-note "don't keep both"): old lossy f64tos deleted; fmt
fprintf_f64_huge "huge"→"9.5e18" (improvement). 5 value-faithful filed-
bug dodges (byte-id, documented): #167/#169/#170/#43/#144. Test 908.
Make test 186/186 incl 990-997 byte-id + combined_ww_fresh.

Drew's strconv 5-fold plan — primary completion (f64tos). f32tos coda
#67 (behind #143); parametric ftosf #64.

lib/strconv/ftos_data.ww

@@ -0,0 +1,104 @@
+// strconv — Ryū float→string lookup tables + bit-count constants.
+// Mirrors ref/hare/strconv/ftos_ryu.ha:159-222 byte-exact. Pure data
+// fold (strconv #106 fold-5): no logic, consumed by ftos.ww's
+// f64computeinvpow5 / f64computepow5 (the Ryū power-of-five cores).
+//
+// File-organisation divergence: Hare keeps these tables INLINE in
+// ftos_ryu.ha. ww splits data from logic into ftos_data.ww (mirroring
+// the stof.ww / stof_data.ww split) — same `package strconv`, so the
+// tables stay visible to ftos.ww with no qualification.
+//
+// Spelling divergences (same as stof_data.ww, candidate #130 + rule-12):
+//   - Hare `const TBL = [...]` → ww module-level `let` (ww has no
+//     module-`const` keyword; the values are never written).
+//   - every literal carries its element-width suffix (`u64`/`u32`):
+//     cstage rejects bare integer literals in `[N]uXX` init while
+//     wwstage accepts them; the suffixed form is the only shape both
+//     stages agree on.
+//   - the [N][2]u64 tables stay faithful 2D (rule-12, not flattened);
+//     the 2D module-level static-init + double-index read landed in
+//     #156 (cbeffea), proven by stof_data.ww's powers_of_ten[596][2]u64.
+
+package strconv;
+
+// ref/hare/strconv/ftos_ryu.ha:159-160. Bit-counts of the split
+// power-of-five tables. Defined u8 (faithful); ftos.ww casts to u32/i32
+// at each use site (Hare promotes a u8 def inside mixed-width arithmetic;
+// ww is strict — explicit cast, project_int_machine_word_derived_limits).
+// The F32_POW5_*_BITCOUNT siblings (ftos_ryu.ha:162-163) land in fold-5b
+// (task #67) with their only consumer, f32todecf32 — omitted here to keep
+// this fold dead-code-free.
+def F64_POW5_INV_BITCOUNT: u8 = 125u8;
+def F64_POW5_BITCOUNT: u8 = 125u8;
+
+// ref/hare/strconv/ftos_ryu.ha:165-181.
+let F64_POW5_INV_SPLIT2: [15][2]u64 = [
+	[1u64, 2305843009213693952u64],
+	[5955668970331000884u64, 1784059615882449851u64],
+	[8982663654677661702u64, 1380349269358112757u64],
+	[7286864317269821294u64, 2135987035920910082u64],
+	[7005857020398200553u64, 1652639921975621497u64],
+	[17965325103354776697u64, 1278668206209430417u64],
+	[8928596168509315048u64, 1978643211784836272u64],
+	[10075671573058298858u64, 1530901034580419511u64],
+	[597001226353042382u64, 1184477304306571148u64],
+	[1527430471115325346u64, 1832889850782397517u64],
+	[12533209867169019542u64, 1418129833677084982u64],
+	[5577825024675947042u64, 2194449627517475473u64],
+	[11006974540203867551u64, 1697873161311732311u64],
+	[10313493231639821582u64, 1313665730009899186u64],
+	[12701016819766672773u64, 2032799256770390445u64],
+];
+
+// ref/hare/strconv/ftos_ryu.ha:183-188.
+let POW5_INV_OFFSETS: [19]u32 = [
+	0x54544554u32, 0x04055545u32, 0x10041000u32, 0x00400414u32, 0x40010000u32, 0x41155555u32,
+	0x00000454u32, 0x00010044u32, 0x40000000u32, 0x44000041u32, 0x50454450u32, 0x55550054u32,
+	0x51655554u32, 0x40004000u32, 0x01000001u32, 0x00010500u32, 0x51515411u32, 0x05555554u32,
+	0x00000000u32,
+];
+
+// ref/hare/strconv/ftos_ryu.ha:190-204.
+let F64_POW5_SPLIT2: [13][2]u64 = [
+	[0u64, 1152921504606846976u64],
+	[0u64, 1490116119384765625u64],
+	[1032610780636961552u64, 1925929944387235853u64],
+	[7910200175544436838u64, 1244603055572228341u64],
+	[16941905809032713930u64, 1608611746708759036u64],
+	[13024893955298202172u64, 2079081953128979843u64],
+	[6607496772837067824u64, 1343575221513417750u64],
+	[17332926989895652603u64, 1736530273035216783u64],
+	[13037379183483547984u64, 2244412773384604712u64],
+	[1605989338741628675u64, 1450417759929778918u64],
+	[9630225068416591280u64, 1874621017369538693u64],
+	[665883850346957067u64, 1211445438634777304u64],
+	[14931890668723713708u64, 1565756531257009982u64],
+];
+
+// ref/hare/strconv/ftos_ryu.ha:206-211.
+let POW5_OFFSETS: [21]u32 = [
+	0x00000000u32, 0x00000000u32, 0x00000000u32, 0x00000000u32, 0x40000000u32, 0x59695995u32,
+	0x55545555u32, 0x56555515u32, 0x41150504u32, 0x40555410u32, 0x44555145u32, 0x44504540u32,
+	0x45555550u32, 0x40004000u32, 0x96440440u32, 0x55565565u32, 0x54454045u32, 0x40154151u32,
+	0x55559155u32, 0x51405555u32, 0x00000105u32,
+];
+
+// ref/hare/strconv/ftos_ryu.ha:213. Divisor/index stride in
+// f64computeinvpow5 / f64computepow5 (ftos.ww). Kept as a def for those
+// arithmetic uses; POW5_TABLE's dimension below must be a literal (cstage
+// rejects a def-named array length — "array length must be an integer
+// literal"; wwstage accepts it but emits an empty DATAW — divergence
+// #167, so the literal `26` is the only shape both stages agree on;
+// matches decimal.ww's `[800]u8` array-dimension-literal precedent).
+def POW5_TABLE_SZ: u8 = 26u8;
+
+// ref/hare/strconv/ftos_ryu.ha:215-222. 5^0 .. 5^25 (the 5^26 entry is
+// commented out in Hare too — it lives implicitly in the SPLIT2 tables).
+let POW5_TABLE: [26]u64 = [
+	1u64, 5u64, 25u64, 125u64, 625u64, 3125u64, 15625u64, 78125u64,
+	390625u64, 1953125u64, 9765625u64, 48828125u64, 244140625u64,
+	1220703125u64, 6103515625u64, 30517578125u64, 152587890625u64,
+	762939453125u64, 3814697265625u64, 19073486328125u64, 95367431640625u64,
+	476837158203125u64, 2384185791015625u64, 11920928955078125u64,
+	59604644775390625u64, 298023223876953125u64,
+];