Merge nucleic/sleek-thistle-egret-fyej into dev

corpus/DIVERGENCES.md

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+# nash ↔ bash divergence log
+
+Live record of behavioral divergences found by the replay harness
+(`replay.py`, corpus in `corpus.jsonl`). Per docs/NASH.md §10.3/§11, each entry
+stays open until fixed in the fork (or upstream) and re-verified by the corpus.
+
+## Open
+
+(none)
+
+## Closed
+
+### D1 — IFS word-splitting applied to literal words (fixed in fork)
+
+- **Found**: M0 corpus run (`ifs-split`), brush-shell-v0.4.0.
+- **Repro**: `IFS=,; set -- a,b,c; echo $1` → bash `a b c`, nash `a`.
+- **Root cause**: brush applied IFS splitting to *literal* words, not just
+  expansion results: under `IFS=,`, `echo a,b,c` printed `a b c` (bash:
+  `a,b,c`) and `set -- a,b,c` received 3 args (bash: 1). POSIX/bash split only
+  the results of parameter/command/arithmetic expansion. Mechanically,
+  `brush-core/src/expansion.rs` had a two-state `ExpansionPiece`
+  (`Splittable`/`Unsplittable`) conflating "field-splittable" with
+  "glob-active", and literal text was marked `Splittable`.
+- **Fix**: added a third piece state `LiteralText` (never field-split, glob
+  chars active) and produce it for unquoted literal text, the no-expansion-chars
+  fast path, and retained-backslash escapes (`// nash:` markers in
+  `expansion.rs`). Corpus back to 100% (94/94); candidate for upstreaming.
+- **Regression tests**: corpus `ifs-split` (+ `glob-all`, `glob-txt`,
+  `cond-pattern`, `case` guard the glob/pattern side); brush compat suite.