Fix repeated-prefix-chain data loss in join_on_conjunctions

[derek73]

Summary

• Unify the i == 0/else branch pair in join_on_conjunctions's conjunction-joining loop into a single block. Pure simplification, no behavior change (verified via characterization tests).
• Fix Repeated prefix chain silently drops part of the name in join_on_conjunctions #208: the prefix-joining loop's value-based pieces.index(prefix) lookup re-found the wrong occurrence when the same prefix word repeated in a chain, silently dropping part of the name (e.g. HumanName("Juan de la de la Vega").last returned "de la Vega" instead of "de la de la Vega"). Replaced with a position-tracked forward scan that absorbs contiguous prefix runs, then extends to the next prefix/suffix boundary — no more .index()/try-except ValueError.
• Small follow-up: corrected a stale comment ("more than 1 rootnames") to match the actual total_length >= 1 condition it describes.
• Cleanup pass (/simplify review): extracted the bundled is_prefix(...) or (i == 0 and total_length >= 1) guard into a named leading_first_name local, so the "exempt a leading piece as a first name" rationale is legible at the call site instead of buried in a compound boolean.
• Further simplification: the contiguous-conjunction join at the top of the method flattened each range into individual indices and deleted them one at a time in reverse. Since group_contiguous_integers already returns non-overlapping ranges, this is now a single loop over the ranges themselves (in reverse), slice-assigning each range directly — consistent with the same pattern the other two loops in this method already use.

Developed via TDD with subagent-driven review (implementer + independent spec-compliance review + independent code-quality review per commit, plus a final whole-branch review). One round of review caught and fixed two duplicate tests along the way. A follow-up /simplify pass (4 parallel cleanup reviews: reuse, simplification, efficiency, altitude) found the rewrite already efficient and free of reuse gaps, and applied the one legible altitude fix above; a couple of other suggestions (merging two nested while-loops, forcing reuse of an unrelated helper) were evaluated and skipped as not actual improvements. One more simplification (the reverse-index-delete loop above) was found afterward on manual inspection.

Known minor side-effect of the fix (found in post-merge-readiness review, not a regression to worry about): the old exception-driven code had an inconsistent suffix boundary — a stale, reused loop index from its except ValueError: pass path could let a prefix run "absorb" past where a suffix token should have stopped it. The new position-tracked scan enforces that suffix-stop boundary uniformly, so a small class of inputs where a suffix-like token sits between prefix words (not at the true trailing position) now classify differently between first/middle than before — e.g. HumanName("aen Sr. aan abu") was first="aen Sr.", middle="" on master, is now first="aen", middle="Sr." on this branch. No words are lost in either version (.suffix is empty in both cases either way, since "Sr." isn't in trailing position). This only showed up in adversarial/synthetic token fuzzing, not in realistic names, and is more consistent with the documented suffix-stop design — but it's an undocumented side effect of the refactor worth calling out for anyone diffing behavior closely.

Fixes #208

Test plan

• python -m pytest -q — 1256 passed, 4 skipped, 22 xfailed (up from the 1248 baseline by exactly the 8 new regression test cases; zero regressions)
• Repeated-prefix-chain bug independently reproduced against the pre-fix commit and confirmed fixed post-fix
• New regression tests in tests/test_prefixes.py: test_repeated_prefix_chain_de_la, test_repeated_prefix_chain_van_der, test_triple_repeated_prefix_chain, test_repeated_prefix_chain_followed_by_suffix
• Two independent fuzz sweeps (20,000 random synthetic tokens; 6,254 name-shaped permutations) against master found zero data-loss regressions — all divergences are either the intended fix or the suffix-boundary reclassification noted above
• Re-ran full suite after each cleanup commit — still 1256 passed, 4 skipped, 22 xfailed