docs: Rational's operators saturate and log; four passages still described UB

[Yaraslaut]

Closes #221.

The defect

rational.hpp's +, - and * detect an overflow before forming the
result, saturate toward the correctly-signed ±INT64_MAX/1, and log at error
naming checkedAdd/checkedSub/checkedMul. Four passages across three
documents still described the pre-saturation behaviour as undefined behaviour.

rational.md made both claims at once — its "Overflow & value-range envelope"
section and its Limitations bullet described silent UB, while line 200, a few
dozen lines below the first, says in bold:

The operators saturate; they never overflow.

A reader deciding whether they need the checked variants got the wrong mental
model twice in the same file.

What changed

Corrected in README.md, docs/spec/util/quantity_type.md, and rational.md
(operator table row, envelope section, the "garbage Rational / UBSan trap"
paragraph, and the Limitations bullet).

The correction is not "it saturates, all is well." A saturated result is
still wrong — clamped and inexact, and nothing in the return type distinguishes
it from an exact one. What changed is the failure mode: bounded and logged
rather than undefined and silent, with the checked variants available for a
caller that must branch on it. I wrote it that way rather than swapping one
over-simplification for another — the old text at least conveyed "don't rely on
this", and a naive fix would have lost that.

Verified against the code, not the issue

• operator*= → mulWouldOverflow() → reportOverflow() + saturateToward()
• operator+= → addWouldOverflow() → same
• reportOverflow() logs at error, naming the checked alternatives

Where the drift shows

tests/test_rational_checked.cpp:154 opens its saturation cases with:

"The saturating operators. Before these, every case below was undefined [behaviour]"

The behaviour was changed and tested; the specs were simply never updated to
match. That's the whole issue in one line.

check	result
[saturate] tests	27 assertions in 7 cases
full suite	20657 assertions in 1170 cases
scripts/check_spec_citations.sh	pass

Documentation only — no code changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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