fix post-TUI typing lag: latch fish query-term off in zellij pane shells

Root cause (proven via live lag-triage capture + deterministic PTY repro):
fish latches feature flags from its startup env before config.fish runs, so
the existing no-query-term settings never applied to zellij-spawned shells.
With query-term on, fish queries the terminal after every command; one reply
zellij fails to relay permanently degrades that fish process's reader.
Subshells were immune (inherited the exported var), which is why the lag
always 'disappeared' when tested in a new shell.

Also adds the lag-triage/unlag/term-probe diagnostic toolkit used to find
this, and upstream_repro.py for filing the fish-shell bug.

docs/CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## 2026-08-29 (root cause found and fixed)
+
+- **Root-caused and fixed the recurring post-TUI typing lag** (fish + Zellij + Ghostty) using a `lag-triage` capture from a live lagging shell plus a deterministic PTY reproduction (`presets/programs/lag-triage/upstream_repro.py`):
+  - **Root cause chain**: (1) fish latches feature flags from its **startup environment**, before `config.fish` runs — so the existing `set -gx fish_features no-query-term` in `shellInit` never applied to the shell that set it, only to its children. (2) Zellij spawns pane shells via `default_shell` with no `fish_features` in the environment, so every pane's fish latched `query-term` **on** (the fish 4.8.1 default; the triage log from the lagging shell confirmed `query-term on` while `$fish_features` was correctly set to `no-query-term`). (3) With query-term on, fish sends OSC 11 + CPR (`\e[6n`) + DA1 (`\e[0c`) after **every** command and waits for replies relayed by Zellij. (4) Reproduced on fish 4.8.1: if the terminal fails to reply during just **one** such cycle — answering everything before and after — that fish process's interactive reader is **permanently degraded** (keystroke echo >3s, never recovers; ~35ms before). In production Zellij drops/mangles a relay during TUI teardown or heavy output (cf. zellij-org/zellij#5158), e.g. after `nh home switch`, nvim, jjui, yazi.
+  - **Why every previous observation finally makes sense**: subshells and `exec fish` were never "resetting" anything — they *inherited* the exported `fish_features=no-query-term` from config.fish, latched query-term off at startup, and were therefore **immune**. The parent zellij-spawned shell never had the variable at startup and stayed vulnerable. Raw keystroke capture in the lagging pane showed instant plain bytes (input path fine) and no stuck terminal modes — the damage was inside the fish process, exactly as the repro shows.
+  - **Fix**: `zellij.nix` now spawns panes through a `fish-no-query-term` wrapper (`export fish_features=no-query-term; exec fish`), so the feature is latched off in every pane shell. Verified: interactive fish through the built wrapper with the real config reports `query-term off`; the PTY repro with `no-query-term` in the environment shows ~35ms echo through all failure phases.
+  - **Correction** to the earlier 2026-08-29 entry: `query-term` does **not** default to off in fish 4.8.1 — it defaults on; it only *appeared* off in non-interactive checks because the user config's `set -gx` takes effect for `fish -c` (no reader latch) but not for interactive shells.
+  - Upstream: fish-shell should bound the reader's wait for query replies instead of degrading permanently (repro script kept at `presets/programs/lag-triage/upstream_repro.py` for filing); Zellij's reply relaying is the trigger (zellij-org/zellij#5158).
+  - `lag-triage` now checks `status features` and calls out `query-term on` as the known root cause, and warns that its `read`-prompt typing tests may not exhibit main-commandline lag (which produced a false "fixed by stage A" in the first capture).
+
 ## 2026-08-29 (later)
 
 - Added a diagnostic toolkit (`lag-triage` / `unlag` fish functions + `term-probe` binary, `presets/programs/lag-triage/`) for the still-recurring post-TUI typing lag in fish + Zellij + Ghostty, instead of another blind fix. Findings that motivated it: