docs(changelog): the unreleased section shortens to the house rule

[tobert]

Pre-release item four. The house rule is that a changelog bullet is one line — the change,
and at most one clause of reason — and that the whole unreleased section gets shortened
before it is retitled for a release.

It had drifted a long way from that. Someone scanning for what changed was reading the
reasoning behind it instead:

	before	after
bullets	47	46
running past one line	41	0
lines	120	95

The reasoning did not disappear — it lives in the pull requests, which is where the rule
says it belongs.

The better find

Shortening surfaced a wrong fact. The kaish bullet said "upgraded to 0.17.1 (from
0.14.1)" while a separate Fixed bullet announced "Bump kaish-kernel to 0.14.1" —
the same cumulative jump stated twice, from a baseline that was not the released one.

v0.3.0 shipped on kaish-kernel 0.14.0:

$ git show v0.3.0:Cargo.toml | grep kaish-kernel
kaish-kernel = { version = "0.14.0", … }

So a reader upgrading from the last release was told the wrong starting point. It now
reads "from 0.14.0", once — and 0.14.1's user-visible fix (the explorer's shell dropping
piped stdin) stays in Fixed as the fix it is, rather than as a second version bump.

This is the same class of error #174's review asked me to double-check on the "from"
baseline. That check confirmed 0.17.0 had never shipped; it did not look at whether
0.14.1 had. It hadn't.

Compression was checked for loss

A shortening pass that quietly drops a change is worse than a long changelog, so this was
verified rather than trusted: diffing the vocabulary of both versions turned up four
things a user would act on that the first pass had dropped. All four are back —

• a slow generation comes back as a job-N handle rather than holding the call;
• base64 content is still the way in for an image that is not a file;
• the four formats write_cas accepts by signature (png, jpeg, gif, webp);
• the twenty-fold cost spread across operations — an exact number is the whole point
  of publishing one.

Everything else that fell out was reasoning, an implementation detail (multipart,
init_image/style_image, ultra/sd3), or a duplicate.

Changelog only — no code.

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