Strip capsules/common/msg.4th -- FABRIC-3.6.md task 0.2

Dead per task 0.1's reachability check (156d1642): zero EXEC sites in
any boot-loaded capsule, and its only two words (HERMES-ACK/
HERMES-NACK) have zero callers anywhere -- messaging.4th's own block
5033 comment already says outright that this file's ACK/NACK
indirection is retired.

mkcapsule --lint capsules/ clean, 38 files / 0 violations (was 39).
Three-arch boot clean: amd64/aarch64/riscv64 all reach [zuse@Hera] ok>,
zero UNKNOWN WORD, dict_hash unchanged from the task 0.0 baseline on
all three -- expected, since the file was never loaded into any VM's
dictionary in the first place.

capsule-reserved.txt and MANIFEST.md's stale block-4055 entry are left
untouched here, per the punchlist's own ordering -- 0.6 returns the
freed block range and 0.5 corrects the manifest once 0.3's strip is
also done.

Authorized by Captain Bob ("Go ahead.").

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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