feat(guard): bump the pin to >=0.3,<0.4 and pin Door C's allow_reserved=False

Measure first, widen after. The 19-fixture guard-surface suite was re-run
against v0.3.4 in a scratch venv before the range moved, and reproduced the
three deltas measured against v0.3.3 exactly, with none added. v0.3.4 is the
tag pinned rather than v0.3.3 because it shipped first and repairs a quadratic
regex (okf._MD_LINK_RE) that sits on Door C's own call path.

Door C now passes allow_reserved=False explicitly. The guard added the keyword
in the 0.3 line and defaults it True for received bundles, which would merge a
sender's index.md / log.md instead of rejecting them. The override keeps the
unconditional reserved-name refusal committed to before the keyword existed,
and the reason is structural rather than a second opinion on the guard's scan:
Door C generates the merged bundle's index.md from what it merged and writes
every merged concept verbatim, so a sender's index.md would be a second and
irreconcilable claim on one path.

This is not a behaviour change for anyone on the previous pin: under v0.2.0
the keyword did not exist and reserved names were refused by construction.

The floor is >=0.3 and not >=0.2 for a measured reason. allow_reserved is
absent in v0.2.0 and present from v0.3.0 onward, checked across all five tags:
a >=0.2 floor would admit a version that raises TypeError on every Door C
import. That measurement also corrects a recorded premise -- the plan said the
keyword "shipped in v0.3.3", which read the first version we ran the suite
against as the version it was introduced in. The conclusion held; the reason
did not, and the reason is what a future bump would have relied on.

test_door_c_pins_allow_reserved_false_against_the_guards_default locks both
halves: that the guard still defaults True, without which the override is a
no-op that would pass forever over nothing, and that Door C overrides it.

586 tests, mypy --strict clean, goldens byte-identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V2v1hrDhrff2H3y2TNJHkF

CHANGELOG.md

@@ -5,6 +5,32 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+- **The guard pin moved to `>=0.3,<0.4`, resolved against `v0.3.4`.** The
+  window is widened only after measurement, never before: the 19-fixture
+  guard-surface suite was run against `v0.3.4` in a scratch venv first, and
+  reproduced exactly the three deltas measured against `v0.3.3` — no new ones.
+  `v0.3.4`'s own fixes are regex-complexity repairs, one of them
+  (`okf._MD_LINK_RE`) on Door C's call path, with no disposition changes.
+
+- **Door C now passes `allow_reserved=False` to `okf.import_bundle`.** The
+  guard added the keyword in the `0.3.x` line and defaults it `True` for the
+  received-bundle path, which would merge a sender's `index.md` / `log.md`
+  instead of rejecting them. Door C overrides it, keeping the unconditional
+  reserved-name refusal committed to before the keyword existed. The reason is
+  structural rather than a second opinion on the guard's scan: Door C generates
+  the merged bundle's `index.md` from what it merged and writes every merged
+  concept verbatim, so a sender's `index.md` would be a second, irreconcilable
+  claim on one path.
+
+  **This is not a behaviour change for anyone on the previous pin.** Under
+  `v0.2.0` the keyword did not exist and reserved names were refused by
+  construction; the explicit argument preserves that outcome across the bump.
+  A consumer sees the same rejections, with the same reasons, before and after.
+
 ## [0.5.0a2] — 2026-07-31
 
 **This is the pre-release the pilots pin. `v0.5.0a1` was tagged and abandoned