perf(dataman): Speed up DatamanCache lookups with a search hint

[JonasPerolini]

This PR changes DatamanCache::loadWait() to scan from the last successful cache hit instead of always starting at slot 0.

This keeps sequential and repeated mission item reads close to O(1) in the common case while preserving the same fallback behavior on cache misses.

Linked to #27707 where the plan is to have the full route in the cache. In this case starting from the last hit can become significant.

[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 24 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +24  +0.0%     +24    .text
    +9.0%     +24  +9.0%     +24    DatamanCache::loadWait()
    +5.9%      +2  +5.9%      +2    DatamanCache::changeUpdateIndex()
    +5.3%      +2  +5.3%      +2    DatamanCache::invalidate()
    -1.6%      -4  -1.6%      -4    FeasibilityChecker::checkTakeoffLandAvailable()
  +0.0%     +72  [ = ]       0    .debug_abbrev
  +0.0%    +136  [ = ]       0    .debug_info
  -0.0%      -8  [ = ]       0    .debug_line
    [DEL]      -5  [ = ]       0    [Unmapped]
    -0.0%      -3  [ = ]       0    [section .debug_line]
  +0.0%     +21  [ = ]       0    .debug_loclists
  +0.0%      +7  [ = ]       0    .debug_rnglists
    +100%      +1  [ = ]       0    [Unmapped]
    +0.0%      +6  [ = ]       0    [section .debug_rnglists]
  +0.0%      +8  [ = ]       0    .debug_str
  -0.3%     -24  [ = ]       0    [Unmapped]
  +0.0%    +236  +0.0%     +24    TOTAL

px4_fmu-v6x [Total VM Diff: 24 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +24  +0.0%     +24    .text
    +9.0%     +24  +9.0%     +24    DatamanCache::loadWait()
    +5.9%      +2  +5.9%      +2    DatamanCache::changeUpdateIndex()
    +5.3%      +2  +5.3%      +2    DatamanCache::invalidate()
    -1.6%      -4  -1.6%      -4    FeasibilityChecker::checkTakeoffLandAvailable()
  +0.0%     +72  [ = ]       0    .debug_abbrev
  +0.0%    +136  [ = ]       0    .debug_info
  +0.0%     +33  [ = ]       0    .debug_loclists
  +0.0%      +3  [ = ]       0    .debug_rnglists
    [DEL]      -3  [ = ]       0    [Unmapped]
    +0.0%      +6  [ = ]       0    [section .debug_rnglists]
  +0.0%      +8  [ = ]       0    .debug_str
  -0.4%     -24  [ = ]       0    [Unmapped]
  +0.0%    +252  +0.0%     +24    TOTAL

Updated: 2026-07-03T18:46:45