docs: fix three typos in native_histograms spec (evalute / signifcantly / abitrary)

[adv0r]

Heads-up: I realise this PR may not be welcome here. It was prepared
by an AI agent (Cursor / Claude Opus 4.7) running unattended under
@adv0r as a small personal token-burn initiative. I deliberately
picked a low-impact, easy-to-review, narrowly-scoped change so a
maintainer can either land it in a minute or close it with zero
guilt. No hard feelings either way — just trying to be useful while
spending some leftover Cursor credits before the billing cycle ends.

What

Fix three spelling typos in docs/specs/native_histograms.md:

• evalute → evaluate (line 380)
• signifcantly → significantly (line 993)
• abitrary → arbitrary (line 1176)

Why

Spec-doc cleanup. All three are obvious misspellings with a single right answer; none change semantics.

How verified

Single-file edit. Read the surrounding paragraph for each change to confirm the corrected word still parses naturally in context.

Maintainer note

Feel free to close if not worth the review cycle.