DOG-004: `dog/WEAVE` emit + scope-bitset + large-file cap

Port the rendering surface onto dog/WEAVE (DOG-003): WEAVEEmitDiff (windowed), WEAVEEmitFull (whole-file), WEAVEEmitMerged (conflict framing), all over an index-bitset scope instead of WEAVEsetfn callbacks. Wire the 16 MiB cap + coarse large-file fallback here, folding DIFF-007. Output stays HUNK records via the existing renderers. Gates on DOG-003.

Input

Context

Today emit takes WEAVEsetfn in_from/in_to callbacks + a per-call decode;
WEAVEEmitFull carves a fixed 65536-tok buffer that overflows (DIFF-007).
A scope is naturally a set of commits[] indices (a rev closure), so the
callbacks collapse to array-membership tests.

Goals

Emit{Diff,Full,Merged} take u32cs from/to (or group sets); same hunk
URIs / navver / scheme as today (DIFF-003/DIFF-004 preserved).
Large-file path: when weave text > 16 MiB, fall back to coarse blob-level
emit and REPORT it (a capped status row), never silently empty (DIFF-007).

Constraints

Output hunks stay HUNK-renderable (HUNKu8sFeedOut); side bits rewritten
eq/in/rm at emit. Byte-parity with current diff/blame/merged where uncapped.

WIP

Design decisions

Reuse weave_overlay_syntax (re-tokenize hunk text for tags; the weave
stores none). Flush-before-overflow guards become cap-driven, not fixed.
WEAVEEmitMerged keeps the byte-equality group collapse (foster/cherry
re-absorption is not a real conflict).

TODOs

WEAVEEmitDiff/EmitFull/EmitMerged over u32cs index scopes.
16 MiB detect + coarse blob-level fallback + a capped report row.
Port the emit tests; add a >16 MiB large-file case that locks DIFF-007.

Blockers and bummers

Gates on DOG-003.

Outcome

(open)