fix: route every transcendental through libm, and combine sigmas with hypot

Follow-on from #41, which added `libm` for `erfc`. Surveying what else
the dependency offers: its unique surface over `std` is `erf`/`erfc`,
`lgamma`/`tgamma` and Bessel functions, and only the first was ever
needed. But the survey found something better than another special
function.

`std`'s `exp` and `ln` delegate to the *system* math library. IEEE 754
specifies the basic operations and `sqrt` exactly and says nothing about
transcendentals, so those differ per platform. Measured here over 200k
inputs:

    exp: 19425/200000 differ from libm (worst 1 ulp)
    log:  9932/200000 differ

Inference is an iterative fixed point, so a one-ULP difference can change
an iteration count and move the answer by more than one ULP. Routing
every transcendental through `libm` makes a fit reproducible across
platforms — a stronger guarantee than `tests/determinism.rs`, which only
covers thread counts.

It costs nothing. `Batch::iteration` measured -2.7% [-5.7%, -0.3%] with
the whole set swapped, and not one golden moved.

Also switches the two places that combined sigmas as
`sqrt(a^2 + b^2)` to `hypot`. Squaring overflows to infinity above
~1.3e154 and flushes to zero below ~1.5e-154 — measured, the naive form
returns `inf` where `hypot` returns 1.41e160 — and `Gaussian`'s
constructors are public, so a caller can reach both ends.

Deliberately not done: rewriting the KL divergence's `ln` of a ratio via
`ln_1p`. The cancellation is real as the ratio approaches one, but
measured absolute error is at most ~1e-11 in a quantity of order 0.4
nats, so it changes nothing.

The invariant is recorded in `CLAUDE.md` and on `erfc`'s own docs, since
nothing enforces it mechanically.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

CLAUDE.md

@@ -97,6 +97,12 @@ History  →  TimeSlice[]  →  Event[]  →  Item[]
   chain underflows to zero, and `ln(0)` is `-inf`.
 - **Colors are contiguous.** `recompute_color_groups` reorders events so each
   color occupies one range; `ColorGroups::groups_are_contiguous` asserts it.
+- **Transcendentals go through `libm`, not `std`.** IEEE 754 pins the basic
+  operations and `sqrt` but says nothing about `exp`/`log`/`erf`, and `std`
+  delegates to the *system* math library — measured, `f64::exp` and `libm::exp`
+  disagree on 9.7% of inputs by one ULP. Since inference is an iterative fixed
+  point, one ULP can change an iteration count. Use `libm::exp` / `libm::log` in
+  inference code; `f64::sqrt` is fine (IEEE specifies it). Tests may use either.
 - **The crate is `#![forbid(unsafe_code)]`.** Keep it that way.
 - **Ingestion order must not change the answer.** Events added one at a time
   must converge to the same fixed point as the same events batched — see