docs(config): reconcile CONFIGURATION.md with the schema, and test that it stays reconciled

[mizu-jun]

What

Config has 36 fields. docs/CONFIGURATION.md covered 13, mentioned one in passing, and documented ten keys that have never existed in the code. Since the TOML parser does not reject unknown keys, all ten parsed silently and did nothing — the document read as though they worked.

This closes the "CONFIGURATION.md inventory PR" item in docs/plans/ui-ux-modernization-v3.md.

Documented but fictional

Doc said	Reality
[terminal] table: alt_screen_buffer, dec_mode_47_1047_1049, osc_window_title, osc_notifications, cjk_width, ime_support	No such table. Replaced with a list of these behaviours stated as unconditional, which is what they are
log_template, placeholders {session}/{date}/{time}	file_name_template, placeholders {session}/{pane}/{datetime}
binary	binary_log
local_forwards (table array with local_port/remote_host/remote_port)	forward_local — an array of "<local>:<host>:<remote>" strings. Same shape for forward_remote
socks5_proxy	No such key
[[serial]]	[[serial_ports]] — and its keys were never documented at all; the section described the command-palette action instead

Also newly documented on [[hosts]]: group and tags, which existed but were missing.

max_log_size — the key was fiction, but so was the behaviour

The document promised automatic rotation at 100 MB. The rotation machinery does exist in pane.rs, but the config-driven path (start_recording_with_config → start_recording_with_options) passes a limit of 0, which disables it.

A log started from nexterm.toml grows without bound. That is now stated plainly, along with the fact that wiring a size limit through to the config is outstanding work. Worth its own issue — this PR only stops the document from promising something that does not happen.

Newly documented (21 sections/keys)

Top-level scalars (api_version, language, leader_key, cursor_style, auto_check_update, colors_follow_system, colors_light, colors_dark, active_profile, plugin_dir, plugins_disabled), plus [cursor], [ui], [animations], [scrolling], [gpu], [quake_mode], [security], [hooks], [[profiles]], [inactive_pane_hsb], [[serial_ports]].

Details worth calling out, because each is a trap for anyone writing a config:

• [gpu] custom shaders — the vertex layouts, and the fact that the fragment output must be premultiplied alpha. A shader written for straight alpha looks wrong wherever it is not fully opaque.
• [quake_mode] — the global hotkey cannot work on Wayland by spec; use nexterm-ctl quake toggle from the compositor instead.
• [security] — plugin_read defaults to deny, not prompt, and prompt is treated as deny because there is no synchronous prompt path for a server-side plugin call.
• [inactive_pane_hsb] — hue is accepted and ignored (a real hue shift needs a shader pass).

Why it drifted, and what now prevents it

Nothing compared the document to the schema. Three tests in the new nexterm-config/tests/doc_matches_schema.rs do:

1. The complete example must parse as Config, with spot-checks on the renamed keys.
2. Every top-level key in that example must be a real Config field — the known-key set comes from serializing Config::default(), not a hard-coded list, so a field added to Config needs no test change. This is the check that catches a [terminal].
3. The ten phantom keys are named explicitly, so copying an old revision back in fails with a file and line number. Prose inside block quotes may still mention them (the document explains why they were removed).

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings — clean through every crate
• cargo test --workspace — all pass, 0 failures
• Both regression tests verified by reintroducing the defect they target: injecting [terminal] into the example fails test 3 with docs/CONFIGURATION.md:221 documents \alt_screen_buffer``; injecting an unknown table fails test 2. Neither passes vacuously
• Bidirectional coverage checked mechanically: every Config field now appears in the doc, and every key documented in a table exists in the code

🤖 Generated with Claude Code

[github-actions]

Coverage report

Generated by cargo llvm-cov (workspace minus nexterm-client-gpu and nexterm-i18n).