feat(#3996): render generated images from manifest-gated portable blobs

[aheritier]

What and why

Revalidate generated-file authorization against the current manifest on every resolution, by owner, path, and workspace root. Session deletion revokes references even when workspace provenance is cached. Invalid POSIX/Windows paths and unknown/external roots are rejected before any lookup. Prefer portable blobs. Fall back only when the optional blob interface is absent or the blob is not found, and only to the owning workspace; other blob errors fail closed. Portable bytes may survive a missing WorkingDir and then use the recorded relative display path, not a verified absolute path. Legacy ordinary-file contents are not integrity-verified. Resolve bytes off the TUI Update path and preserve adjacent text.

Part of #3996. Review this PR against its immediate parent, #4028, rather than the aggregate stack against main.

Commit inventory

Head: 950d801ff3a2eb4ee5aaf56265bf6fe8faa54110; parent SHA: 8eed51894134f104fa449f2cfbe7e510a22d3956.

• 8c2a76fad28771134b95cd1d797221403ea4f3f8 — feat: render generated images through workspace manifest authorization
• 950d801ff3a2eb4ee5aaf56265bf6fe8faa54110 — feat: restore generated media from portable session blobs

Validation

Build, test compilation, owning-package tests and the named fixture passed at this PR head.

Exact deterministic fixture command:

go test -v -count=1 ./pkg/runtime ./pkg/tui/components/message ./pkg/tui/components/messages ./pkg/tui/page/chat -run 'TestResolveGeneratedFile|TestAppendAssistantMediaPreservesStreamedTextRendering|TestLoadFromSession.*Media|TestUpdateAssistantMedia|TestAssistantMedia|Test.*GeneratedMedia'

Matched top-level tests: pkg/runtime: 52; pkg/tui/components/message: 4; pkg/tui/components/messages: 5; pkg/tui/page/chat: 3.

Deterministic scope: the named local fixture exercises this PR boundary with disposable configuration/stores and fake or loopback providers as applicable. Every listed package ran nonzero matching top-level tests.

Deferred/live scope: No active database or remote runtime was exercised. The final stack head passed build, lint, full tests, an uncached full suite, focused race tests and documentation checks in disposable environments. Remote CI is tracked by the checks below; no new paid-provider or active-database validation was run.

[aheritier]

👋 This PR has merge conflicts with the base branch. Please rebase or merge the latest base branch and resolve them. I've moved it to draft and added status/needs-rebase; it'll be picked back up automatically once the conflicts are cleared.

[aheritier]

👋 This PR has merge conflicts with the base branch. Please rebase or merge the latest base branch and resolve them. I've moved it to draft and added status/needs-rebase; it'll be picked back up automatically once the conflicts are cleared.