docs(errors): document exception hierarchy and enrich Errors reference page

[justinjoy]

Resolves #167.

Improves the Errors reference documentation in two atomic commits, produced via a consensus-gated implementation flow (architect+critic guidance → implementer → reviewer → architect+critic audit → consensus).

Changes

Unit 1 — 199372a — docstrings + presence test

• Documents the 5 previously-undocumented exception classes (InvalidIRError, ExecError, WirelogIOError, CompoundSaturatedError, CompoundBusyError) and expands the terse ones; each cites its WIRELOG_ERR_* code and when it is raised.
• ExecError documented as the catch-all (invalid args, closed/null handles, missing relations).
• ParseError gains an Attributes: section for its best-effort line/column/source payload.
• Fixes module-docstring miscount ("Two" → "Three" PyreWire-only exceptions).
• Adds a self-discovering test asserting every WirelogError subclass in the module has its own non-trivial docstring.

Unit 2 — 8b9c152 — enrich docs/reference/errors.md

• Intro overview, wirelog_error_t code→exception mapping table, a "PyreWire-only errors (not produced by check())" section, an exception-handling example (public imports), and ::: directives for check() / error_string().
• All 11 per-class blocks preserved.

Accuracy

raised when descriptions cross-referenced against bundled wirelog 0.51.0 (0c6e0cd) and the real raise sites. The consensus audit corrected three docstring-accuracy defects before merge:

• WirelogVersionError — described the wrong same-named class; corrected to the capability/symbol gate at program.py:163.
• WirelogModeError — "insert batch" → session-level mode exclusivity (step/snapshot/query).
• WirelogIOError — "via check()" → direct raise via io_adapter.py.

Verification

• flake8 / black / isort clean; errors.py mypy clean
• pytest: errors 27/27, full suite 91.24% coverage
• mkdocs build --strict: passes (0 errors)

Follow-ups (non-blocking, out of scope)

• WirelogInternError docstring documents reverse-intern lookup only, but intern.py:46 also raises on forward-intern failure (pre-existing).
• Two distinct classes named WirelogVersionError (_core/errors.py vs _ffi/_loader.py) — consider consolidating.