Fix systemic -Wmissing-field-initializers across src/test_runner/modules/ (3010 -> 0)

TestCase gained a trailing `contract` field (WordContract) at some point
after all 20 test-module files' compound literals were written -- every
single TestCase/WordTestSuite initializer in the tree (sentinels, real
entries, and per-suite entries) omitted it, producing ~3010 warnings on
every build. CLAUDE.md's own documentation claimed this was isolated to
one file (vocabulary_words_test.c); a full audit found it systemic
across all 20 files.

Fixed mechanically: added the missing `{0}` trailing initializer
everywhere. Semantically a no-op -- C99 already zero-fills unlisted
trailing struct fields, so this only silences the diagnostic, changes
no behavior. Verified: all three architectures (amd64/aarch64/riscv64)
build clean, remaining warning count unchanged (30, matching the other
three known -Wno-error-exempted classes: unused-parameter, sign-compare,
plus mkcapsule.c's stringop-truncation which was never actually gated
by this policy -- it's a separate host tool with no -Werror at all).

.claude/CLAUDE.md corrected to describe the actual -Wno-error= exemption
list (four classes, not "build with -Wall -Werror" unconditionally) and
the real current warning inventory.

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

capsules/BLOCK_MAP.md

@@ -1,5 +1,5 @@
 # Capsule Block Manifest — Auto-generated
-<!-- Generated by mkcapsule --manifest  2026-08-19T00:14:53Z -->
+<!-- Generated by mkcapsule --manifest  2026-08-19T01:42:10Z -->
 <!-- DO NOT EDIT — re-run mkcapsule --manifest to refresh.  -->
 <!-- Hand-written justifications and immutability notes live  -->
 <!-- in MANIFEST.md alongside this auto-generated index.      -->