Document sender efficiency and reliability improvements

Requirements.md

@@ -32,6 +32,8 @@ Function names below are C++ references. Rust naming/layout may differ, but the
   - sender sample cadence 1 Hz.
   - sender batch cadence 30 s.
   - sync-request cadence 15 s while unsynced.
+  - sender retransmits reuse cached encoded payload bytes for same inflight batch.
+  - sender ACK receive window is adaptive from airtime + observed ACK RTT, with expanded second window on miss.
 - Receiver behavior:
   - decode/reconstruct sparse timestamps.
   - ACK accepted batches promptly.
@@ -44,6 +46,9 @@ Function names below are C++ references. Rust naming/layout may differ, but the
   - `WEB_AUTH_REQUIRE_AP=true`
 - Web and display time rendering:
   - local timezone from `TIMEZONE_TZ`.
+- Sender diagnostics:
+  - structured sender diagnostics are emitted to serial debug output only.
+  - diagnostics do not change LoRa payload schema or remap payload fields.
 - SD logging:
   - CSV columns include both `ts_utc` and `ts_hms_local`.
   - history parser supports both current (`ts_utc,ts_hms_local,p_w,...`) and legacy (`ts_utc,p_w,...`) layouts.
@@ -56,6 +61,7 @@ Function names below are C++ references. Rust naming/layout may differ, but the
 - `AckDown` payload fixed length `7` bytes:
   - `[flags:1][batch_id_be:2][epoch_utc_be:4]`
   - `flags bit0 = time_valid`
+  - sender acceptance window is implementation-adaptive; payload format stays unchanged.
 - `BatchInput`:
   - fixed arrays length `30` (`energy_wh`, `p1_w`, `p2_w`, `p3_w`)
   - `present_mask` must satisfy: only low 30 bits used and `bit_count == n`
@@ -107,11 +113,15 @@ Function names below are C++ references. Rust naming/layout may differ, but the
   - parse OBIS values and set meter data fields.
 - `bool meter_read(MeterData&)`
   - compatibility wrapper around poll+parse.
+- `void meter_get_stats(MeterDriverStats&)`
+  - expose parser/UART counters for sender-local diagnostics.
 - Internal parse helpers to preserve numeric behavior:
-  - `parse_obis_ascii_value`
+  - `detect_obis_field`
+  - `parse_decimal_fixed`
+  - `parse_obis_ascii_payload_value`
   - `parse_obis_ascii_unit_scale`
   - `hex_nibble`
-  - `parse_obis_hex_u32`
+  - `parse_obis_hex_payload_u32`
   - `meter_debug_log`
 
 ## `src/power_manager.cpp`
@@ -345,6 +355,7 @@ These functions define end-to-end firmware behavior and must have equivalents:
 - Queue and sample batching:
   - `batch_queue_drop_oldest`
   - `sender_note_rx_reject`
+  - `sender_log_diagnostics`
   - `batch_queue_peek`
   - `batch_queue_enqueue`
   - `reset_build_counters`
@@ -384,6 +395,7 @@ These functions define end-to-end firmware behavior and must have equivalents:
 - LoRa TX pipeline:
   - `send_batch_payload`
   - `send_batch_ack`
+  - `invalidate_inflight_encode_cache`
   - `prepare_inflight_from_queue`
   - `send_inflight_batch`
   - `send_meter_batch`
@@ -405,6 +417,7 @@ These functions define end-to-end firmware behavior and must have equivalents:
 
 - Preserve wire compatibility first:
   - LoRa frame byte layout, CRC16, ACK format, payload schema v3.
+  - sender optimization changes must not alter payload field meanings.
 - Preserve persistent storage keys:
   - Preferences keys (`ssid`, `pass`, `mqhost`, `mqport`, `mquser`, `mqpass`, `ntp1`, `ntp2`, `webuser`, `webpass`, `valid`).
 - Preserve timing constants and acceptance thresholds: