feat(v1): add SubNav breadcrumb component (Phase 1 - Foundation PR 3)

[vitorvasc]

First foundation primitive after the v1 router scaffold (#470). SubNav is the breadcrumb row that sits beneath the navbar on every inner page.

Contributes to #84 and #370.

What's in this PR

• New <SubNav crumbs={...} actions={...} /> at src/v1/components/layout/sub-nav.tsx:
  • crumbs: BreadcrumbItem[] is required, actions?: ReactNode is an optional right-side slot for page-level controls
  • Last crumb renders as a non-link <span aria-current="page">. Earlier crumbs render as <Link> when href is set, otherwise as plain text
  • Returns null when both crumbs and actions are empty, so route layouts can mount it unconditionally (home passes nothing, inner pages pass their breadcrumb)
  • Lucide ChevronRight separator.
• Scoped CSS partial at src/v1/styles/sub-nav.css uses the .td-subnav class. Selectors only match elements rendered by <V1App />, so it stays inert in the legacy bundle
• src/v1/styles/index.css picks up one @import for the new partial plus a comment update
• Not yet mounted on any route

What's not in this PR

• Mounting SubNav on any route. Each page-level PR consumes it as it ships

Verification

• bun run typecheck clean
• bun run lint clean
• bun run test src/v1/components/layout/sub-nav.test.tsx passes 5/5 (covers empty, crumbs-only, actions-only, crumbs+actions)
• bun run test passes 842/842
• bun run format no reformatting needed

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Co-authored with Claude Opus 4.7.

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[Copilot]

Pull request overview

Adds a new <SubNav> breadcrumb component to the v1 sub-app as a foundation primitive. It's not yet mounted on any route; later page-level PRs will consume it.

Changes:

• New SubNav component with crumbs (required) and optional actions slot, returning null when both are empty.
• New scoped sub-nav.css partial under .td-subnav, imported from src/v1/styles/index.css.
• Vitest coverage for empty/crumbs-only/actions-only/combined render paths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ecosystem-explorer/src/v1/components/layout/sub-nav.tsx	New SubNav component with breadcrumb landmark and actions slot.
ecosystem-explorer/src/v1/components/layout/sub-nav.test.tsx	Unit tests covering the four render permutations.
ecosystem-explorer/src/v1/styles/sub-nav.css	Scoped .td-subnav styles for the row.
ecosystem-explorer/src/v1/styles/index.css	Imports the new partial; updates header comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.