docs: P0 review: server instructions, handbook, local files, fail closed

docs/mcp-interface.md: the connect text and its budgets, the handbook
(tool and resources), annotations, failing closed, import_local_file's
roots and refusals, the set_w9 path rule and where TIN-bearing files may
go. docs/testing.md: the P0 laws and the tests that check them.
docs/architecture.md: the journal format gate and the server's
fail-closed store lock.

Round-2 review (2026-10-16): mcp-interface.md lists all 26 destructive
tools (unlink_receipt, unlink_attachment and correct_asset were missing)
and openWorldHint false on every tool; the connect text states the
parties rule and the exact receipt rule, and how it fits its budget; the
lint covers every refusal; the import roots refuse /, the home folder and
folders holding the books or a key, the open is a component walk, every
missing or outside path gets one refusal, and the TIN-file registry and
text scan (and what they can't catch) are described. architecture.md:
the frozen envelope, the lenient next-line prev, no repair of newer
books. testing.md: the new laws and tests. verification.md and
parity.md: P0 notes (no new law to prove, no status moved).
The TIN text check, narrowed (2026-10-16; rules.md B-26,
mcp-interface.md): text is refused for a TIN the vault holds, in its
common renderings, or a stand-alone printed SSN or EIN shape, so bank
exports go in; the oracle a vault comparison gives is recorded as a
known limit for P7. testing.md: the P0 mutation run of the final diff,
per crate, and how each survivor was resolved (killed by a named test,
removed by simpler code, or argued equivalent).
The probe bound (rules.md B-26, mcp-interface.md): rule (a) answers
for at most two candidates a filing, so finding a TIN from its last four
takes on the order of 50,000 refused filings; the open limit now names
that path, closed by P2's access log and P7's human-only TIN channel.
testing.md: the law and its tests; tin_files.rs mutated again, 74 of
74 caught.

Review round 3: rules.md B-28 (four-digit years, amounts a line can
carry, checked products, bounded recurring runs) and B-26/B-27 updates
(BOM-less UTF-16, when set_w9 records); mcp-interface.md matches the
code (25 destructive tools, the five idempotent ones, no reverse_entry
receipt checklist, strictly above the threshold, no tool text names
wanderbooks.toml, dot names and roots inside the books refused, the new
settings lists); testing.md lists the new laws and this round's
mutation run.

Review round 4: rules.md B-28 says the bounds are for new commands
only, replay accepts every line an earlier version accepted, and derived
figures refuse rather than panic; mcp-interface.md documents plan_error
and the nullable totals; testing.md lists the old-books contracts and this round's
mutation run (core, store, MCP, config, GUI).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

docs/parity.md

@@ -49,6 +49,19 @@ No other statuses.
 Workflow/UI parity (QuickBooks/Xero reference) gets its own catalog when the
 egui phase starts; semantics come first.
 
+### P0 guard rails (2026-10-16)
+
+P0 (`plan/served.md`) changed no feature in the catalog above, so no
+status moved and the floor stands. It hardened what exists: an older
+version refuses books from a newer one as newer, not corrupt (rules.md
+B-27); the MCP server fails closed after an internal error, and no input
+reaches one (dates are four-digit years, amounts at most a line's, B-28);
+local files
+are read only from the import roots, never the books, the vault or its
+key; no file with a full TIN is filed into the books (B-26); the agent is
+told the rules in force in a short connect text and a handbook, and no
+agent text points at the books' files, git or the repo.
+
 ## Capability parity (AI-first phase)
 
 Semantic parity (above) is about *correctness*: the engine computes the