P3-09: context-aware Herdr popup plugin

[dvzrd]

Summary

• Add --herdr-context and optional root; explicit roots win, invalid plugin context silently falls back to the current directory.
• Add a pure core JSON parser with explicit serde_json dependency using the existing locked version.
• Ship a Linux/macOS 80% popup manifest and bindable action invoking HERDR_BIN_PATH, plus operator setup docs.
• Start the unreleased dogfood section after merging P3-09/P3-10: herdr spike and Phase 3 execution decisions #127. Keep P3-09 active pending the shipping-plugin manual pass.

Verification

• scripts/check.sh passes (fmt, clippy, tests, links, rumdl, ADR-0006 dependency guard).
• Added parser/root precedence and fallback tests, manifest contract checks, and action argv/quoting/failure checks without touching the live Herdr session.
• Manual linking/popup checks await operator permission. Installed dogfood binary remains v0.1.3; no release cut.

P3-10 follows in a separate PR.

[dvzrd]

Manual retest: correct collection, Help/Esc/Ctrl+Enter/q and clean dismissal pass, but images and Mermaid graphics fail (no graphics protocol). Explicit image config and 2s timeout do not help. Raw popup response confirms Kitty OK, but has no cell-size reply; separate 14t/18t/16t queries are also unanswered. ratatui-image 11.1.0 drops the confirmed protocol when cell metrics/PTY pixel fallback are unavailable, returning default Halfblocks/10x20. Evidence recorded in the spike doc; acceptance remains blocked and this PR stays draft. Temporary test plugin unlinked; plugin list is empty again. Installed v0.1.3 and user config/keybindings unchanged. scripts/check.sh rerun successfully after docs updates.

[dvzrd]

Further investigation: a probe delayed 300ms still returns Halfblocks/default 10x20. Tagged Herdr 0.9.0 source starts popups with zero pixel geometry, omits PluginPaneOpen from public/client geometry-change classifiers, and normal client-shell drawing passes resize_panes=false. The classifier omission remains in inspected v0.9.3/master, so upgrading alone is not an evidenced fix. Spike doc now records source links, a proposed host-side initial-geometry + invalidation fix, and regression requirements (immediate child query, actual metrics, no sleep/resize, CLI/action/keybinding paths). No patched Herdr build or forced protocol attempted. Second diagnostic plugin unlinked; installed binary/config unchanged. Full gate passed again.

[dvzrd]

Local host patch prepared (uncommitted/unpublished) against upstream master 5da0a01e, exported to /tmp/wiki-reader-herdr-popup-geometry.patch with validation notes. It initializes popup PTY and virtual-terminal metrics before child startup, invalidates geometry on plugin opens, and uses request-owner/tab-controller cell metrics. Five added regressions pass; disabling the core fix makes four fail. Native suite: 3713 pass / 12 skipped. Native fmt/clippy, architecture, Python maintenance and Bun integration/docs checks pass. Full upstream just check remains unverified (just missing; Windows cross-validation not run). No upstream implementation PR permitted for the authenticated external account; no host install/restart or patched client connected to the stable server. P3-09 remains draft pending real graphics acceptance. wiki-reader scripts/check.sh passes after recording these findings.