build_dynaword: opt-in preserve_id / preserve_created flags (split from #19)

The shared-build half of the studia_bas contribution (#19), split out for independent review per that PR's review item 2. Small and self-contained:

• src/build_dynaword.py — two opt-in per-source registry flags: preserve_id keeps the intermediate row's id in the canonical output; preserve_created stamps the per-record created/date/published value instead of the source-level default. Sources without the flags are gated and stamped exactly as before — src/test_build_dynaword_flags.py pins that backward-compat behavior (3 tests, passing).
• .gitignore — __pycache__/*.pyc plus the build-input/scratch patterns of the studia_bas fetch pipeline (PDF cache, .jsonl.zst intermediates, probe output). Personal tooling lines from the original branch were dropped.

Companion source PR: the studia_bas data PR linked above. Merge order is free — the flags are inert registry keys until the flags land; the committed shard is pre-built either way.