fix: isolate reusable bundle plans from session mutations

[Brian Krabach (bkrabach)]

Modules can add runtime defaults to their configuration while mounting. Foundation's shallow mount-plan copies allow these changes to alter the reusable Bundle, so a later child can inherit its parent's working directory. Reusing a PreparedBundle also passes the same mutable plan into multiple sessions.

Return an independent nested configuration from Bundle.to_mount_plan(), and copy the prepared plan for each create_session() before injecting observability events or mounting modules. Session-specific configuration changes remain local; declared settings and observability subscriptions are preserved.

Validation:

• All seven new regressions fail on unchanged main and pass with this fix. They cover nested configuration across mount-plan sections, future child composition, repeated prepared sessions with different directories, and per-session event subscriptions. The prepared-session regression uses a real Core session/coordinator with a simulated mutating module initializer.
• 106 focused bundle/session/observability/mention tests pass.
• Full suite: 2,268 passed, 50 skipped, one pre-existing failure in TestVerifyModuleType.test_non_isinstance_object_with_mount_passes. Its isinstance check raises because Core's Tool protocol is not runtime-checkable; the same node fails against unchanged Foundation3a5aa568 with both Core112 Python and installed Core1.6.1. This PR does not alter that unrelated test or interface.
• Disposable integration with the current unmodified filesystem module verifies real child reads/writes/edits and Bash all use the child directory, and the parent Bundle stays unchanged. Also verified with the companion filesystem fix.
• Ruff checks on changed files and wheel/sdist builds pass.

No host deployment or live-session changes. Integration used Core112 Python source at e2cf2a6f with the installed Core1.6.1 native extension; it was not a native rebuild at that commit.