Implement browser write accountability receipts

[mdheller]

Purpose

Implement BearBrowser support for SourceOS Mutation and Evidence Accountability, anchored to SourceOS spec PR #96:

SourceOS-Linux/sourceos-spec#96

Required behavior

BearBrowser must emit browser write accountability receipts that distinguish:

• browser core profile state;
• history / Places-style storage;
• favicons;
• cookies and permissions;
• session restore;
• per-origin storage;
• service-worker/cache state;
• download/cache writes;
• sync state;
• extension storage;
• hidden/system/policy add-ons;
• diagnostics;
• profile repair or migration.

No-visible-extension guardrail

If extension_inventory_state=none_visible, the browser must not attribute write pressure primarily to extension storage unless hidden/system/policy add-on evidence is attached.

Acceptance criteria

• A write-pressure event emits a receipt with actor chain, object/path class, operation, resource cost, policy, evidence quality, and causal parent fields.
• The receipt can represent browser_actor_class and extension_inventory_state.
• Missing database path/origin/extension evidence downgrades evidence quality.
• Tests include a no-visible-extension fixture and reject extension-primary attribution without evidence.