emit-portable: windowed relexing for regex-context lexers (S5a, #57)

[johnsoncodehk]

Summary

Slice 5a of #57 (portable incremental edit). Extends windowed relexing (#64, #65) to lexers with regex-vs-division context (regexCtx, no templates — e.g. regexjs):

• The regex-context lexer carries eight cross-token state variables. Restoration strategy per class: prevText/prevKind/bpText/hasPrev/hasPrev2 are rebuilt directly from the token stream at the restart anchor; lastClose/lastBang are recorded per token (their post-emission values) and read back; the parenHead boolean stack is reconstructed backward from per-token paren depth (pd) plus a head bit (hd) recorded on ( tokens — porting the reconstructParens argument from the JS incremental lexer.
• Resync tightens accordingly: besides the S3 token-identity conditions, the old token must match the live scan on paren depth, stack contents (element-wise), lastClose/lastBang, and previous-token text. Any mismatch keeps scanning instead of adopting the old suffix.
• streamEq self-check now compares the recorded state fields too, so an adopted suffix with stale context state cannot survive silently.
• javascript (regexCtx + templates) and templatejs (templates) keep the full-relex fallback — the template-stack slice is next (S5b).
• Gate: 6 new regexjs scenarios, including both context-flip directions (deleting a turns two divisions into one regex literal; if→f turns a regex back into divisions), an unbalanced-( insertion that must refuse resync, and a )-anchored append whose relex outcome hinges on the restored lastClose bit.

Validation

• node test/portable-targets.ts — calc 8/8, javascript 5/5 (still full-relex), regexjs 6/6, envspec 8/8, × 3 targets, all streamEq true
• node test/check.ts emit-parity — 5/5; node test/newline-mode.ts — 29/29; npx tsc --noEmit — clean
• Mutation tests (×2): dropping the lastClose restore fails the )-anchor scenario; dropping the paren-stack comparison fails the unbalanced-( scenario — both reverted before commit
• Independent adversarial probe (6 attacks × 3 targets): block-comment-interior edit, depth-3 paren stack, ( inside a regex body (not a real paren), division after ], non-head ) anchor with lastClose=false, two-batch state inheritance chain — all parity with oracle-derived expectations