tri pr preflight: add the own-language budget and the developer-home-path rules the t27o loop runs by hand #11070

Goal

Give tri pr preflight (specs/tri/pr/preflight.t27, #10190) the two checks the t27o loop still runs by hand before every push: the own-language foreign-line budget compiled from the merge base, and the developer-home-path scan. A lane then learns about both from one command, before CI, instead of from a red check.

Why

Both gaps cost CI runs on 2026-10-18/19: #10538 failed the required own-language check at +9/-0 hand lines; #10646 turned scan red for every later PR with nine /Users/... test literals; #11023 repeated it with one line of prose in a skill. The preflight already covers the seal, vacuity, duplicate-bodies, types-ratchet and ledger-row rules, but none of these two. The loop replayed the budget gate by hand on eight PRs tonight (compile gen/c/policy/own_language.c from the base, pipe git diff --numstat --no-renames BASE...HEAD), which is exactly a step-and-rule for this spec.

Boundary

• specs/tri/pr/preflight.t27
• gen/rust/tri/pr/preflight.rs
• .trinity/seals/pr_TriPrPreflight.json
• cli/tri/src/prcheck.rs

User Scenarios

• Given a branch whose diff adds 9 hand-written Rust lines and deletes none, when tri pr preflight runs, then it fails the budget rule and prints the gate's own output as evidence.
• Given a branch that adds a line naming a developer home directory, when tri pr preflight runs, then it fails the home-path rule and prints the offending file and line.
• Given a branch that changes neither, when it runs, then both rules pass and the exit code is unchanged.

Requirements

• FR-001: The preflight MUST run the budget gate compiled from the merge base's gen/c/policy/own_language.c, not from the branch.
• FR-002: The preflight MUST run tools/check_devhome_paths.py and fail a branch that it fails.
• FR-003: Both rules MUST fail closed: output that is not the shape the spec expects reads UNREAD, never a pass.
• FR-004: The rule MUST live in the spec; hand Rust in cli/tri/src/prcheck.rs MUST NOT grow (own-language net <= 0).

Success Criteria

• t27b test specs/tri/pr/preflight.t27 and t27c test-report pass with 0 vacuous passes, and t27c seal --verify matches.
• The own-language check reports net <= 0 for the PR.
• On PR t27o --check: t27b test judges first, t27c test-report only when t27b cannot (Closes #10525) #10538's head (+9/-0), the new rule reports a failure.