fix(changelog): report a public-dependency change once, and only claim a break when proven

[upbqdn]

Motivation

Two problems in the public-dependency reporting path, both visible in the draft a curator
has to hand-edit.

One change produced two bullets. A dependency whose types are reachable in a crate's
public API got both the Migrated to ... line that every requirement change emits and a
second Public dependency ... line from the reachability join:

### Changed
- Migrated to `ext_dep 0.30.0`.
- Public dependency `ext_dep` upgraded to `0.30.0` (major); it appears in this crate's public API, so this is a breaking change.

A changelog entry is per change and written for the user adapting to it, so this left the
curator merging the two by hand every time.

An undecidable change was counted as a break. #11 added the unknown classification
for a requirement that changed without moving the version it starts at, and routed it into
the reachability join so it would be surfaced. But the join's output feeds
totals.public_dep_breaking and the verdict, so widening a version ceiling
(>=0.1, <0.3 -> >=0.1, <0.4) produced verdict: breaking and a nonzero exit — failing
a consumer's CI over a change nothing shows to be incompatible.

Solution

Fold the note into the Migrated to line:

### Changed
- Migrated to `ext_dep 0.30.0`; its types appear in this crate's public API, so downstream users must upgrade `ext_dep` in lockstep.

And separate the two classes throughout:

	provably incompatible	undecidable
changelog wording	must upgrade in lockstep	check whether downstream users are affected
totals.public_dep_breaking	counted	not counted
verdict / exit code	breaking, exit 1	unchanged
JSON class	breaking	review
summary section	red, "incompatible"	yellow, "compatibility unclear"

So an undecidable change is still surfaced — the point of routing it through the join —
without asserting a break or demanding a major version bump.

The new class field is documented in both the README's --json reference and the inline
schema zc --help prints, so neither describes a shape the tool no longer emits.

Tests

• The reachable incompatible case asserts the folded line and guards against a second
  bullet appearing beside it.
• A new case covers the undecidable arm end to end: exit 0, verdict: ok,
  public_dep_breaking: 0, a class: review entry, the review wording in the folded
  line, and the absence of any break assertion.
• Branch coverage for the external Migrated to line itself, which the fold rewrites in
  place.

Rebased onto main now that #11 has landed; supersedes #12, which GitHub auto-closed when
its stacked base branch was deleted.

Ran locally: shellcheck -S warning zc tests/run.sh, bash -n zc, bash -n tests/run.sh, tests/run.sh (137 tests, 0 failures).