AP_Terrain: Add const to locals

[Ryanf55]

Purpose

Add local const to the AP terrain libraries. Pulled out from another branch.

Why

• https://isocpp.org/wiki/faq/const-correctness#retrofitting-const
• https://isocpp.org/wiki/faq/const-correctness#const-and-type-safety

[rmackay9]

Looks correct to me. Probably best to also clarify what testing has been done

[peterbarker]

LGTM

[Ryanf55]

I'm flying with this patch in SITL with terrain following enabled. It works (flies without hitting terrain)

[Ryanf55]

From durandal:
https://github.com/ArduPilot/ardupilot/actions/runs/12014292966/job/33489748482?pr=28734#step:10:25

Binary Name      Text [B]     Data [B]     BSS (B)      Total Flash Change [B] (%)      Flash Free After PR (B)
---------------  -----------  -----------  -----------  ----------------------------  -------------------------
arducopter-heli  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      144768
antennatracker   0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      614640
arducopter       0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      144312
ardurover        0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      298360
arduplane        0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      146784
blimp            0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      59[26](https://github.com/ArduPilot/ardupilot/actions/runs/12014292966/job/33489748482?pr=28734#step:10:27)92
ardusub          0 (0.0000%)  0 (0.0000%)  0 (0.0000%)  0 (0.0000%)                                      354100