Add Svelte GUI, FIT encoder and README

Standalone binary embeds the frontend, avoiding the dev-server dependency
that made the window fail to load.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/fit/src/encode.rs

@@ -35,7 +35,8 @@ pub const PROFILE_VERSION: u16 = 21_205;
 pub const DATA_TYPE: &[u8; 4] = b".FIT";
 
 /// FIT base type identifiers. The high bit marks an endian-sensitive type; the
-/// low 5 bits are the type number.
+/// low 5 bits are the type number. Variant names are the FIT type names.
+#[allow(missing_docs)]
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 #[repr(u8)]
 pub enum BaseType {
@@ -54,7 +55,9 @@ pub enum BaseType {
     Byte = 0x0D,
 }
 
-/// One encoded field value, carrying its own base type and width.
+/// One encoded field value, carrying its own base type and width. Variant
+/// names mirror the FIT base types they encode to.
+#[allow(missing_docs)]
 #[derive(Debug, Clone, PartialEq)]
 pub enum Value {
     Enum(u8),
@@ -176,9 +179,8 @@ impl Message {
         self.fields.is_empty()
     }
 
-    /// The definition-message shape of this message: (field number, size, base
-    /// type) per field. Two messages sharing a shape can share a definition.
-    fn shape(&self) -> Vec<(u8, u8, u8)> {
+    /// The definition-message shape of this message.
+    fn shape(&self) -> Shape {
         self.fields
             .iter()
             .map(|(n, v)| (*n, v.size(), v.base_type() as u8))
@@ -186,6 +188,10 @@ impl Message {
     }
 }
 
+/// A definition-message shape: `(field number, size in bytes, base type)` per
+/// field. Two messages sharing a shape can share a definition.
+type Shape = Vec<(u8, u8, u8)>;
+
 /// Accumulates data records and emits a complete FIT file.
 ///
 /// Definitions are cached per local message type, so a definition is re-emitted
@@ -195,7 +201,7 @@ impl Message {
 pub struct FitEncoder {
     data: Vec<u8>,
     /// Cached definition shape per local message type (0..16).
-    defs: [Option<(u16, Vec<(u8, u8, u8)>)>; 16],
+    defs: [Option<(u16, Shape)>; 16],
     message_count: usize,
 }
 
@@ -345,18 +351,48 @@ pub fn verify(bytes: &[u8]) -> Result<(), VerifyError> {
 /// Why [`verify`] rejected a file.
 #[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
 pub enum VerifyError {
+    /// Shorter than the smallest legal file (header plus CRC).
     #[error("file is {0} bytes, too short to be a FIT file")]
-    TooShort(usize),
+    TooShort(
+        /// Actual file length.
+        usize,
+    ),
+    /// Byte 0 is neither 12 nor 14.
     #[error("header size {0} is neither 12 nor 14")]
-    BadHeaderSize(u8),
+    BadHeaderSize(
+        /// The declared header size.
+        u8,
+    ),
+    /// Bytes 8..12 are not `.FIT`.
     #[error("data type signature is {0:?}, expected \".FIT\"")]
-    BadSignature([u8; 4]),
+    BadSignature(
+        /// The bytes found where the signature should be.
+        [u8; 4],
+    ),
+    /// The header's data size does not match the bytes actually present.
     #[error("header declares {declared} data bytes but the file carries {actual}")]
-    DataSizeMismatch { declared: usize, actual: usize },
+    DataSizeMismatch {
+        /// Data size from the header.
+        declared: usize,
+        /// Data bytes actually present.
+        actual: usize,
+    },
+    /// The header CRC does not check out.
     #[error("header CRC is {stored:#06x}, computed {computed:#06x}")]
-    HeaderCrc { stored: u16, computed: u16 },
+    HeaderCrc {
+        /// CRC read from the file.
+        stored: u16,
+        /// CRC computed over bytes 0..12.
+        computed: u16,
+    },
+    /// The trailing file CRC does not check out.
     #[error("file CRC is {stored:#06x}, computed {computed:#06x}")]
-    FileCrc { stored: u16, computed: u16 },
+    FileCrc {
+        /// CRC read from the end of the file.
+        stored: u16,
+        /// CRC computed over header plus data.
+        computed: u16,
+    },
 }
 
 #[cfg(test)]