telemetry: add timestamp index for latency sample gap correction

[snormore]

Summary of Changes

• Add a companion TimestampIndex PDA account that records per-write-batch timestamps, enabling accurate wall-clock reconstruction even when agents experience downtime gaps
• Implement onchain program (new instruction + write-time updates), Go SDK instruction builders, device telemetry and internet latency collector submitters, and read-only deserialization in Go/Python/TypeScript SDKs
• Include timestamp reconstruction helpers using binary search (single sample, O(log m)) and single-pass cursor (batch, O(n+m)) in all three SDK languages

RFC: rfcs/rfc18-telemetry-write-timestamp-index.md

Closes #877

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	28	+1378 / -7	+1371
Tests	10	+337 / -5	+332
Docs/RFC	2	+232 / -0	+232
Other	1	+5 / -0	+5

~70% core logic, ~17% tests, ~12% RFC/docs. Note: this PR exceeds the ~500-line guideline due to cross-language SDK parity (Go + Python + TypeScript) and the onchain program changes — splitting further would leave incomplete functionality.

Key files (click to expand)

• smartcontract/programs/doublezero-telemetry/src/state/timestamp_index.rs — New TimestampIndex account type: header struct, serialization, and 10K-entry ring buffer
• smartcontract/programs/doublezero-telemetry/src/processors/telemetry/initialize_timestamp_index.rs — New instruction: creates the companion PDA derived from the samples account
• smartcontract/programs/doublezero-telemetry/src/processors/telemetry/write_timestamp_index.rs — Appends a (sample_index, timestamp) entry on each write batch
• smartcontract/programs/doublezero-telemetry/src/processors/telemetry/write_device_latency_samples.rs — Optionally accepts 4th account (timestamp index) for backward compat
• sdk/telemetry/go/state.go — Go read-only SDK: DeserializeTimestampIndex + ReconstructTimestamp(s)
• sdk/telemetry/python/telemetry/state.py — Python read-only SDK: TimestampIndex.from_bytes + reconstruct helpers
• sdk/telemetry/typescript/telemetry/state.ts — TypeScript read-only SDK: deserializeTimestampIndex + reconstruct helpers
• controlplane/telemetry/internal/telemetry/submitter.go — Device telemetry submitter: derives timestamp index PDA, passes on writes, initializes after new account creation

Testing Verification

• Rust onchain program tests cover initialization, write-time appending, ring buffer wrap-around, and backward compatibility (writes without timestamp index account)
• Cross-language fixture tests verify binary compatibility: Rust generates .bin/.json fixtures, Go/Python/TypeScript deserialize and assert field values match
• Timestamp reconstruction unit tests in all three SDK languages validate binary search, single-pass cursor, entry boundary transitions, and empty-entries fallback

[martinsander00]

In both submitters, when InitializeTimestampIndex fails, the code logs "writes will proceed without it" but retries the write with TimestampIndexPK` still set

[martinsander00]

will approve, not sure if nik or karl want to take a looks since they were involved in the initial convo in #877