fix(spec): stop to_dict dropping stated number starts and titles

[Alpha9463]

Summary

Stops to_dict() from silently changing a sheet on the way to a file. A float number start drew S1.0 and read back as S1; a falsy annotation title was dropped and the box came back re-measured. Number starts now accept integers only, and a stated annotation title is written as stated, so reading back a non-text title fails loudly instead of silently.

Related Issues

Closes #533

Changes

• Number starts: Flowsheet.stream_number_start, line_number_start and loop_number_start become validated properties. A float (even 1.0), Decimal, string or bool raises TypeError naming the attribute, at construction and on assignment. Any integer type, including numpy.int64, is stored as an int. to_dict() compares with the default exactly as before, which is now exact because the values are always integers.
• Annotation titles: _write_annotation writes the title whenever it differs from the default "", rather than only when truthy. A title of 0, False or None is written as stated and refused by from_dict() with a SpecError naming annotations[N].title. Rendering is unchanged.
• Tests: tests/test_stated_values.py (27 tests) covers both halves, for Annotation and TableBox.
• Docs: one sentence in docs/api.md on integer number starts, plus [Unreleased] entries in CHANGELOG.md.

A correction to the issue text: Annotation(title=0) never drew a "0" title. The band is drawn only for a truthy title while the box is sized from str(title), so it drew an untitled box sized as if titled. That rendering is unchanged here; the round trip now refuses it instead of silently resizing the box.

How to Test

python3 -m pytest -q tests/test_stated_values.py tests/test_spec.py tests/test_loops.py tests/test_stream_numbering.py
python3 -c "from pandid import Flowsheet; Flowsheet('x', stream_number_start=1.0)"   # TypeError
python3 -m pytest -q
python3 -m ruff check .
python3 -m ruff format --check tests
python3 -m mypy pandid
python3 -m pyright pandid

Evidence

• Full suite: 11,521 passed, 188 skipped.
• Lint and types: ruff, test formatting, mypy and git diff --check are clean. Pyright reports 0 errors plus the existing __all__ warning, and 0 errors on the new test file.
• Drawings unchanged: frames, routes and validation findings are identical to main for 46 drawings (the 21 examples as authored and with placement stripped, plus four benchmark sheets).

Checklist

• Base branch is main and the branch is mergeable
• Commit subjects follow Conventional Commits; tracked work links its issue
• Self-reviewed the full diff on the Files changed tab
• Relevant local checks pass; all CI jobs pass before merge
• Documentation and examples updated if behaviour changed
• Before/after drawings included if rendered output changed (no rendered output changed)
• Blocking review feedback resolved, if review was requested

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