feat(perms): add SSE subscribe_bundles to BuilderTxCache

[Evalir]

Summary

• Adds a new sse cargo feature that enables signet-tx-cache/sse, gated behind perms so SSE deps (eventsource-stream, serde_json) stay opt-in
• Adds subscribe_bundles() to BuilderTxCache which connects to the /bundles/feed SSE endpoint with bearer auth, using the upstream subscribe_inner helper to avoid code duplication
• Bundles are a protected route, so the method fetches the OAuth token from SharedToken and passes it as Some(&secret) — unlike subscribe_transactions/subscribe_orders which pass None

Test plan

• cargo check --features sse compiles
• cargo check --features perms still compiles (no regression)

🤖 Generated with Claude Code

[Evalir]

• feat(perms): add SSE subscribe_bundles to BuilderTxCache #131 👈 (View in Graphite)
• main

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[Fraser999]

Seems like we're missing this feature from the manifest. I expect we want something like:

sse = ["perms", "signet-tx-cache/sse"]

[Evalir]

yep i messed up a conflict marker here :D