M0.5 Decision: lock the stack and revise the time model from M0 numbers

[melvincarvalho]

Part of #1 (M0). Closes the benchmark phase.

Goal

Synthesize M0.1-M0.4 into a written decision and a measured (not modeled) time estimate.

Acceptance

• Chosen: WASM secp build, hashing approach, OPFS layout, SAB-or-not.
• A revised end-to-end time model per tier, using real numbers.
• Re-validate the epic (Roadmap: browser-native Bitcoin node (epic) #1) perf table against measurements; correct it where the model was wrong.
• Go/no-go on Tier 2 (full from-genesis) feasibility on commodity hardware.