doc: exception handling mechanism and CRuby contrast

[sisshiki1969]

Summary

Adds doc/exception_handling.md — a walkthrough of monoruby's exception mechanism contrasted point-by-point with CRuby, capturing the findings from the Exception#backtrace work (#896).

The document's through-line is laziness: monoruby stores the minimum at raise time and defers the expensive work (exception-object materialization, backtrace string formatting) until something actually asks for it, which is what keeps the raise path — including the control-flow pseudo-exceptions that reuse the same machinery — cheap.

Sections:

• MonorubyErr (the in-flight Rust error) vs the lazily-materialized Ruby exception object (take_ex_obj), including GC rooting of smuggled Values.
• Two families of MonorubyErrKind: real exceptions vs control-flow pseudo-exceptions (return/break/throw/retry/redo/fatal), and why the latter are dispatched before any trace capture, so they pay nothing for backtraces.
• handle_error as the per-frame unwinder, the per-method exception table (get_exception_dest), ensure deferral (the deferred-unwind stack), and $! restoration.
• Backtrace construction — the key contrast: incremental tuple capture on unwind, the catch-time caller walk (complete_backtrace_for_rescue) explained as the last coherent snapshot point (why not eager-at-raise, why not lazy-at-#backtrace), lazy string formatting + memoization in the /backtrace ivar, and why loop's internal StopIteration pays almost nothing (caught at the Rust level below the bytecode-rescue path).
• Fatal errors, top-level reporting, a file map, and a hot-path cost table.

Each section pairs the monoruby behavior with the corresponding CRuby mechanism (eager-at-raise rb_backtrace_t snapshot, catch_table entries, exc_setup_cause, uncatchable fatal).

Test plan

Documentation only — no code changes. File references and cited line numbers were verified against the current tree.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.51%. Comparing base (8753ca6) to head (0a1e6ba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #897      +/-   ##
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+ Coverage   90.50%   90.51%   +0.01%     
==========================================
  Files         189      189              
  Lines      123227   123227              
==========================================
+ Hits       111521   111535      +14     
+ Misses      11706    11692      -14     

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