Cut 19.0.0 and bump the package version in pyproject.toml, CITATION.cff,
and uv.lock.

Major: five breaking changes ship. The `tablassert` console command now
requires the optional `[cli]` extra, since `cyclopts` and `rich` left the
base install (#187). Level-one normalization redefines fullmap keys as
cleaned Unicode-lowercase, Porter2-normalized, byte-ordered token sets and
moves the database to schema v6, so every schema-v5 fullmap is rejected and
must be rebuilt (#171, #172, #174, #175, #176). `build-fullmap --aria2c`
is gone -- aria2c is used automatically whenever the `[aria2]` extra is
installed (#178) -- and `--taxon-allowlist` is gone because the built-in
top-100 experimental-taxon allowlist now applies to every build, guarding
both reuse paths by `META.taxon_allowlist` identity (#153). Logging now
requires the `[log]` extra and is fully disabled without it (#152).

Features: `tablassert.net` is a new stdlib-only transient/permanent
classification and retry seam shared by the agent and the BABEL downloader
(#163, #164), with one bounded jittered retry layer across the inner agent,
reflexion, and judge (#167) and machine-readable `error_code` values on
skipped checkpoint records (#166). `build-kg` gained an automatic ephemeral
shared-prefix TCode cache keyed by content-addressed XXH64 op digests
(#155-#159, #161). The `--distill` corpus became schema-uniform v2 with a
sibling `outcomes.ndjson`, a deterministic `RewardConfig`-tunable reward,
and the new zero-dependency `tablassert distill-weigh` command (#180-#183).
`CLASS_FIELD_OVERRIDES` grants the DAKP sparse qualifier stack on the pinned
association classes (#188).

Fixes: `distill-export` partitions its corpus by content and unions the
schema across record files, so a mixed v1/v2 directory no longer CastErrors
or silently stringifies a column (#184); BABEL retry warnings survive the
loguru sink; PMC downloads are idempotent, atomic, and bounded-parallel
(#165); reflexion and judge prompts are hard-bounded (#168).

Changelog:
- versioned the Unreleased section as 19.0.0 - 2026-09-15
- merged three duplicate `### Added` blocks into one
- moved the two `BREAKING:` entries from `### Changed` into
  `### Breaking Changes` and added a `**Migration:**` note to all five
- added PR links to all 17 entries, which carried none
- added the missing entries: the distill v2 corpus/reward/distill-weigh
  stack (#180-#183), the distill-export schema-drift fix (#184), the
  TCode run-cache detail (#155-#161), and the README badge removal (#154)

Docs: none needed here. Every shipped doc change landed with its own PR,
and the docs source-of-truth gate passes against the bumped tree.

Testing:
- make check -> exit 0
- uv run ruff check . -> All checks passed!
- uv run pyright -> 0 errors, 0 warnings, 0 informations
- uv run pytest -> 1622 passed, 52 skipped
- cargo test --manifest-path rust/Cargo.toml -> 156 passed, 0 failed
- cargo clippy --all-targets -- -D warnings -> clean
- uv lock --check -> resolved 169 packages, lock current
- uv run mkdocs build --strict -> exit 0
- docs SSOT + CLI coverage after the changelog edit -> 204 passed