docs(spec): record Phodopus successor direction in plugin runtime RFCs (CTX-0043)

[Xuepoo]

Priority: P1 | Area: docs | Labels: docs, P1, area:docs, area:lua | Milestone: v0.1.0 | RFC: OQ-009 / OQ-014 | Task: CTX-0043

What

Records the Phodopus successor runtime direction in the plugin-ecosystem runtime RFCs, per bitty-docs ADR 0012 (accepted 2026-09-20). Adds direction only; promotes no status and rewrites no accepted evidence.

Phodopus (https://github.com/bitty-terminal/phodopus) is the accepted successor Lua runtime. bitty-lua still depends on piccolo = "0.3.3"; the migration is scheduled, not landed. The accepted mlua/Lua 5.4 baseline and every dated measurement remain intact.

Changed files

• runtime/lua-runtime-rfc.md — ADR-0004 / technology-strategy clause now reads piccolo was the watch-list candidate, with the plugin-VM successor direction recorded as Phodopus (ADR-0012). Successor paragraph updated to keep the accepted baseline and current piccolo pin explicit.
• runtime/isolation-resource-rfc.md — one clearly-marked successor-direction note; OQ-009 out-of-scope clause names ADR-0012; the RC table status paragraph gains a successor-direction sentence; the RC-1 row policy cell notes the successor enforcement engine while keeping the piccolo Fuel/wall evidence at d67a65b.
• runtime/plugin-host-runtime-rfc.md — successor-direction note; ADR-0005 out-of-scope clause names ADR-0012; the A.5 Send bullet records that Phodopus preserves the single-owner-thread boundary.

Evidence preserved (not rewritten)

Nothing touched in the dated measurement evidence: d67a65b (2026-08-27), CTX-0037 PR #68, CTX-0040, CTX-0049, CTX-0050, the RC-1..RC-11 ceilings, the P0 Review Sign-off rows, and the historical measurement blockquotes remain byte-for-byte as accepted.

Honest-status caveat

• Direction only: no accepted contract changed, no status promoted, no compatibility promise, and no migration performed.
• piccolo 0.3.3 remains the accurate bitty-lua pin until an implementing task lands.
• The Phodopus Plugin Runtime (Candidate) page stays draft; this PR does not promote it.

Gate evidence (this worktree)

$ just check
fmt-check     All matched files use Prettier code style!
markdownlint  Summary: 0 issues in 0 files
links         Validated 70 Markdown files without network access
metadata      Validated 70 Markdown files without network access
language      Validated 70 Markdown files without network access
agents        Validated 70 Markdown files without network access
hygiene       Validated 70 Markdown files without network access
svg           (xmllint) ok
actionlint    clean
exit 0

Review ask

• Independent docs review by a different agent (per workspace lifecycle).
• Confirm the successor direction is additive, the piccolo pin is still accurate, and no dated measurement evidence or ADR history was rewritten.