fix(record): keep mixed-unit sequences as text and refuse repeats into a unique member

[devin-ai-integration]

What and why

Two gaps in how a sequence-valued tool output fits a record definition, found after #580 landed:

• classifySequence settled a quantity element and a plain Integer element to one Real member through numericPair, so (300 [K], 301) recorded as (300, 301) with no tempsUnit companion — the measured element lost its unit. A sequence carries one shared unit, so a mixed quantity/plain sequence has no faithful typed list literal: it now falls back to the String text spelling, as a sequence of mixed enumeration types already does.
• An existing record definition whose multi-valued member is unique (the default) passed checkExisting for a sequence with repeated values, and the generated record then failed validation against that member. record.Feature gains Unique (read through semantics.IsUnique, which follows an inherited nonunique), classifySequence notes a repeated element literal, and checkExisting refuses f.multi && f.repeated && decl.Unique with the usual into remedy. Generated definitions are ordered nonunique and keep admitting repeats.

How it was verified

• internal/exec/analysis/record/sequence_compat_test.go: quantity+Integer sequence records as ScalarValues::String with no unit companion; unique [0..*] existing member + repeated sequence → refusal naming into; [0..*] ordered nonunique + repeated → accepted and parses; unique member + non-repeating sequence → accepted.
• internal/frontend/repl/record_sequence_compat_test.go: recordAttributes reports Unique for Real[0..*], not for nonunique, and follows nonunique through :>>.
• gofmt -l ., go build ./..., go vet, go test -count=1 ./internal/exec/analysis/record ./internal/frontend/repl ./cmd/sysml, scripts/changelog.py check.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

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