feat: support rc-based hotfix version segment

[joshua-temple]

Problem

The version model only understands vX.Y.Z and vX.Y.Z-rc.N. A version carrying an in-flight hotfix segment (vX.Y.Z-rc.N.hotfix.M) fails version.Parse. Because the orchestrator reads the next environment's version into Calculator.CalculateNext, an environment holding a hotfix version would break orchestration of the environment below it. Hotfix-aware version handling is a prerequisite for the rest of the hotfix capability.

Fix

Extend internal/version:

• Add Hotfix int to Version (sentinel -1, matching PreRelease).
• Parse the new vX.Y.Z-rc.N.hotfix.M shape via a nested optional regex group, so a hotfix segment is only valid after an rc segment. vX.Y.Z-hotfix.M, missing/non-numeric hotfix numbers, and the wrong separator are rejected.
• String() round-trips the dotted form.
• Base()/BaseVersion()/StripRC strip the hotfix segment (the published semver is vX.Y.Z).
• WithHotfix(m) and NextHotfix() helpers (first hotfix is hotfix.1).
• Compare() gives strict precedence: rc.2 < rc.2.hotfix.1 < rc.2.hotfix.2 < rc.3.
• CalculateNext computes from the rc base and ignores any .hotfix.M on the next env version, so a diverged environment cannot break orchestration of the one below it.

Per the locked design, the dotted segment applies only to rc-based (still-in-flight) versions; a published-base hotfix is a normal patch bump handled elsewhere, so no vX.Y.Z-hotfix.M shape is added here.

Verification

• go build ./...: success
• go test ./internal/version/... ./internal/promote/... ./internal/orchestrate/...: 207 passed
• golangci-lint run ./internal/version/... ./internal/promote/...: no issues

New tests: TestParse_HotfixSegment, TestParse_RejectsMalformedHotfix, TestVersion_Ordering_HotfixBetweenRCs, TestCalculateNext_NextEnvHoldsHotfixVersion, TestStripRC_HotfixVersion, and promote-side TestStripRCSuffix_HotfixVersion. Existing non-hotfix tests are unchanged in intent (expected-value literals gained the -1 hotfix sentinel only).