Plan / Interface: canonical mecatui status surfaces

[jbeda]

Plan / Interface

Defines the acceptance contract for removing mecatui's duplicate nil-source status/header generators. The canonical path is the existing stock-template or configured status source.

• No source: display only a safe short session handle in the header, with no generated right-side footer; keep mandatory chrome.
• Debug sessions: reserve space for a fixed target warning ahead of generated header content and retain the privacy disclosure even with an empty source or fatal view.
• Stock context meters: report unknown and estimated occupancy truthfully at all responsive widths.
• Do not recreate the legacy eight-tier formatting ladder or its byte-for-byte goldens.

This PR is a plan only; it contains no implementation. Human merge approves the behavior and interface contract before implementation.

Validation

• Acceptance-plan checker: passed (advisory warning about no TestADR/TestInvariant proof names; scenario proofs are named).
• Checker regression fixtures: passed.
• task docs: passed (strict links and generated references).
• Advisory design review: debug source-width reservation, terminal-safe IDs, and narrow/fatal disclosure visibility incorporated.