resolvewalk had no per-block scoping: inner-block `let i: u64`
persisted past the block end and shadowed the outer `let i: i32`,
which then false-positived as u64→i32 not-assignable on the next
reference. The TODO at the N_LET tail explicitly deferred per-block
scoping; this discharges it.
N_BLOCK case mirrors cstage cmd/wcc/check.c:1559-1566: save c.cur,
newscope under saved, walk body via n.list, restore. Sole exit is
the return after restore — push/pop balanced by structure.
All 5 selfhost main.combined.ww files (wwdump, w6c, w6a, w6l, ww)
now resolve clean via wwdump_ww -r. Reviewer's independent probe
across every .combined.ww outside ref/ confirmed no cascade: only
selfhost/cmd/ww went 1→0 (the targeted bug); the other 14
files-with-errors are pre-existing assignability/match-typing
issues unrelated to scope resolution.
Discharges TODO at N_LET tail. Same-scope dup detection
(`let a=1; let a=2;` in one block) stays queued behind #11.
Unblocks #50.
Hojun-Cho
01775f2ccc