feat: add stream rate changes and decentralized pair pricing

[Ay-obami]

Summary

This PR consolidates Ay-obami's three assigned StreamFi contracts issues into one reviewable change set.

#619 — Prospective stream rate changes

• adds sender/operator-authorized change_rate
• rejects non-positive, unauthorized, ended, cancelled, and overflowing changes
• checkpoints already-accrued value so a new rate never retroactively reprices prior streaming
• preserves pause/resume accounting by shifting active rate checkpoints with paused time
• emits a dedicated sequenced rate_chg event

#620 — Atomic top-up + extension event

• replaces the generic top-up event for top_up_and_extend with one dedicated top_ext event
• includes old/new balance and old/new end time in one payload
• preserves one event-sequence increment for the single logical state transition
• adds event-shape regression coverage

#695 — Decentralized pair-price aggregation

• changes Address and Symbol pair feeds from last-writer-wins to timestamped per-reporter reports
• caps each pair at five active authorized reporters
• returns the median of reports no older than 15 minutes
• excludes revoked reporters and stale observations
• keeps legacy scalar pair-price keys as migration fallback for pre-upgrade deployments
• preserves direct and reciprocal pair-price APIs
• adds median, staleness, reporter-cap, and symbol-pair tests

Verification

• cargo +stable test -p drip-stream --lib — 130/130 passed
• cargo +stable test -p drip-oracle --lib — 81/81 passed
• cargo +stable clippy -p drip-stream -p drip-oracle --all-targets -- -D warnings — passed
• git diff --check — passed

A full-workspace local run was interrupted only because the workstation filled its build volume; generated build artifacts were then cleaned. Upstream CI remains the complete workspace gate.

Closes #619
Closes #620
Closes #695

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