terminal: use one capability policy for image widgets and backend emission

[subinium]

Priority: P2
Evidence: Executed public PtyBackend/kitty_image output comparison
Baseline: v0.23.0 source 37933f9, audited 2026-09-05. Native execution/compile probes used Rust 1.98.0 on aarch64 macOS unless explicitly stated otherwise.

Problem and Reproduction

A valid2x2 RGBA fixture renders half-blocks on generic xterm and a Kitty payload on Kitty. Kitty identity plus SLT_DISABLE_KITTY=1 emits only the 16-byte synchronized-update wrapper, with neither image nor fallback. Zellij inheriting Kitty identity behaves the same. Widget-side env inference overrides backend policy.

src/context/widgets_display/rich_output.rs:458 · src/context/widgets_display.rs:790 · src/terminal.rs:1372

Scope

• Resolve protocol support/disable/remote/multiplexer policy once and share it between preparation and emission.
• Select a usable fallback when a payload will not be emitted.
• Audit Sixel/iTerm variants for the same split decision.

Expected Effect

Avoid blank image regions when users disable a protocol or run inside a multiplexer.

Acceptance Criteria

• Assert visible fallback content, not merely absence of Kitty/Sixel escape bytes.
• Test disable-vs-force precedence, inherited TERM_PROGRAM, SSH and supported multiplexer policies.
• Default generic terminals and explicit supported-protocol paths retain their existing output.
• Follow the repository quality gates and the evidence rules in tracking: post-v0.23 correctness, resource efficiency and WASM completion #366; record actual results rather than treating a proposed test as passed.

Boundaries and Dependencies

Byte-emission evidence is not a promise that every physical emulator renders each protocol.

No blocking implementation dependency identified; coordinate shared code with the tracker.

Related closed work: #329, #289, #357. This is a scoped follow-up at the audit baseline, not a claim that every prior fix is absent.

Tracking: #366. This issue records unresolved work, not an implemented fix.

[subinium]

v0.24.0 Disposition

The same capability policy now controls image widget preparation and terminal emission, with environment/subprocess and protocol regressions. Windows raw capability reads are disabled in favor of conservative fallback, rather than consuming console records. Physical Kitty/Sixel/iTerm rendering across real emulator and multiplexer combinations remains unverified; keep this acceptance item open rather than treating protocol tests as screenshots.

Implementation and evidence mapping: PR #433. Release v0.24.0 and its 20/20 release workflow are verified. This issue stays open only for the remaining acceptance/investigation described above; publication alone does not establish it.