feat(simulation): load canonical 16-bit grayscale heightmap.png + downsample (D-202 #963)

Replace the atlas_body_heightmaps DB-BLOB loader with a per-body file
loader (D-202 amendment): heightmap.rs reads the 16-bit grayscale
heightmap.png (via the new png dep) from the body's terrain_reference
path, normalizes to f32 [0,1], and rejects RGB so a reliefmap can't be
misread as elevation. Adds BodyHeightmap::downsample (box-average,
deterministic) so Layer 1 drops the high-res stored heightmap to the
512x256 working resolution. sea_level becomes body metadata carried
alongside, not in the PNG. 4 loader tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/Cargo.lock

@@ -2,6 +2,12 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "adler2"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "320119579fcad9c21884f5c4861d16174d0e06250625266f50fe6898340abefa"
+
 [[package]]
 name = "ahash"
 version = "0.8.12"
@@ -190,7 +196,7 @@ dependencies = [
  "bevy_reflect",
  "bevy_tasks",
  "bevy_utils",
- "bitflags",
+ "bitflags 2.10.0",
  "bumpalo",
  "concurrent-queue",
  "derive_more",
@@ -325,6 +331,12 @@ dependencies = [
  "thread_local",
 ]
 
+[[package]]
+name = "bitflags"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
+
 [[package]]
 name = "bitflags"
 version = "2.10.0"
@@ -462,6 +474,15 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "crc32fast"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9481c1c90cbf2ac953f07c8d4a58aa3945c425b7185c9154d67a65e4230da511"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "critical-section"
 version = "1.2.0"
@@ -563,7 +584,7 @@ version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "89a09f22a6c6069a18470eb92d2298acf25463f14256d24778e1230d789a2aec"
 dependencies = [
- "bitflags",
+ "bitflags 2.10.0",
  "block2",
  "libc",
  "objc2",
@@ -654,6 +675,15 @@ version = "2.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
 
+[[package]]
+name = "fdeflate"
+version = "0.3.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e6853b52649d4ac5c0bd02320cddc5ba956bdb407c4b75a2c6b75bf51500f8c"
+dependencies = [
+ "simd-adler32",
+]
+
 [[package]]
 name = "find-msvc-tools"
 version = "0.1.9"
@@ -666,6 +696,16 @@ version = "0.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1d674e81391d1e1ab681a28d99df07927c6d4aa5b027d7da16ba32d1d21ecd99"
 
+[[package]]
+name = "flate2"
+version = "1.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "843fba2746e448b37e26a819579957415c8cef339bf08564fe8b7ddbd959573c"
+dependencies = [
+ "crc32fast",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "foldhash"
 version = "0.2.0"
@@ -886,13 +926,23 @@ version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
+[[package]]
+name = "miniz_oxide"
+version = "0.8.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1fa76a2c86f704bdb222d66965fb3d63269ce38518b83cb0575fca855ebb6316"
+dependencies = [
+ "adler2",
+ "simd-adler32",
+]
+
 [[package]]
 name = "nix"
 version = "0.31.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "225e7cfe711e0ba79a68baeddb2982723e4235247aefce1482f2f16c27865b66"
 dependencies = [
- "bitflags",
+ "bitflags 2.10.0",
  "cfg-if",
  "cfg_aliases",
  "libc",
@@ -946,7 +996,7 @@ version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2a180dd8642fa45cdb7dd721cd4c11b1cadd4929ce112ebd8b9f5803cc79d536"
 dependencies = [
- "bitflags",
+ "bitflags 2.10.0",
 ]
 
 [[package]]
@@ -1035,6 +1085,19 @@ version = "0.3.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7edddbd0b52d732b21ad9a5fab5c704c14cd949e5e9a1ec5929a24fded1b904c"
 
+[[package]]
+name = "png"
+version = "0.17.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "82151a2fc869e011c153adc57cf2789ccb8d9906ce52c0b39a6b5697749d7526"
+dependencies = [
+ "bitflags 1.3.2",
+ "crc32fast",
+ "fdeflate",
+ "flate2",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "portable-atomic"
 version = "1.13.1"
@@ -1175,7 +1238,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b91f7eff05f748767f183df4320a63d6936e9c6107d97c9e6bdd9784f4289c94"
 dependencies = [
  "base64",
- "bitflags",
+ "bitflags 2.10.0",
  "serde",
  "serde_derive",
 ]
@@ -1186,7 +1249,7 @@ version = "0.32.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7753b721174eb8ff87a9a0e799e2d7bc3749323e773db92e0984debb00019d6e"
 dependencies = [
- "bitflags",
+ "bitflags 2.10.0",
  "fallible-iterator",
  "fallible-streaming-iterator",
  "hashlink",
@@ -1305,6 +1368,7 @@ dependencies = [
  "crossbeam-channel",
  "econ-sim",
  "pathfinding",
+ "png",
  "rand",
  "rand_chacha",
  "rayon",
@@ -1336,6 +1400,12 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
 
+[[package]]
+name = "simd-adler32"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "703d5c7ef118737c72f1af64ad2f6f8c5e1921f818cdcb97b8fe6fc69bf66214"
+
 [[package]]
 name = "slab"
 version = "0.4.12"
@@ -1747,7 +1817,7 @@ version = "27.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "afdcf84c395990db737f2dd91628706cb31e86d72e53482320d368e52b5da5eb"
 dependencies = [
- "bitflags",
+ "bitflags 2.10.0",
  "bytemuck",
  "js-sys",
  "log",