analyzer: add four new warning diagnostics

Extends the analyzer with the warnings listed under "Error message
polish" in docs/future-work.md:

- W0102 (existing): now also fires for `while true { ... }` and
  `loop { if cond { continue } }`. `continue` is explicitly not
  counted as an exit — the loop still spins forever.
- W0105 (new): palette declarations whose sub-palette first-bytes
  disagree. The NES mirrors $3F10/$3F14/$3F18/$3F1C onto $3F00/
  $3F04/$3F08/$3F0C, so a 32-byte sequential write silently
  overwrites the background universal colour; the grouped
  `universal:` form auto-fixes this so only flat `colors: [...]`
  declarations can trip it.
- W0106 (new): a call at statement position whose callee has a
  declared return type — the value is silently dropped.
- W0107 (new): `fast` variables with fewer than three observed
  reads+writes, which don't justify holding a scarce zero-page
  slot. Leading-underscore names are exempt.

All four are warnings only — no IR, codegen, or runtime changes,
so every committed example ROM rebuilds byte-identical and no
emulator goldens flip. Tests added in src/analyzer/tests.rs.

One legitimate W0106 surfaces on examples/platformer.ne (the
`resolve_enemy_hit` helper uses early-return for control flow
but its return value is discarded by the caller); fixing it
would shift the ROM and flip goldens, so it is left in place
as an informational hint rather than a hard fix.

https://claude.ai/code/session_01MaNVcDmK9gsspRkdxowQAM

src/errors/diagnostic.rs

@@ -39,6 +39,9 @@ pub enum ErrorCode {
     W0102, // loop without break or wait_frame
     W0103, // unused variable
     W0104, // unreachable code after terminator, or unreachable state
+    W0105, // palette sub-palette universal mismatch (mirror collision)
+    W0106, // implicit drop of non-void function return value
+    W0107, // `fast` variable rarely accessed (wastes zero-page slot)
 }
 
 impl fmt::Display for ErrorCode {
@@ -62,6 +65,9 @@ impl fmt::Display for ErrorCode {
             Self::W0102 => "W0102",
             Self::W0103 => "W0103",
             Self::W0104 => "W0104",
+            Self::W0105 => "W0105",
+            Self::W0106 => "W0106",
+            Self::W0107 => "W0107",
         };
         write!(f, "{code}")
     }
@@ -70,7 +76,13 @@ impl fmt::Display for ErrorCode {
 impl ErrorCode {
     pub fn level(self) -> Level {
         match self {
-            Self::W0101 | Self::W0102 | Self::W0103 | Self::W0104 => Level::Warning,
+            Self::W0101
+            | Self::W0102
+            | Self::W0103
+            | Self::W0104
+            | Self::W0105
+            | Self::W0106
+            | Self::W0107 => Level::Warning,
             _ => Level::Error,
         }
     }