Fix offset-indirect, field-level endian, and aligned materialization

Resolves review #003 findings M1, M2, L1, and L2.

- offset-indirect (L1 + M2 arm): rework read_*_indirect to same-buffer
  absolute offsets (safetensors-style, per the metatensor model) and add
  write_*_indirect. Wire the encoding dispatch into engine.read_field/
  write_field and the aligned materializer. Previously the encoding was
  laid out but never read back.
- field-level endian override (M1): thread field.effective_endian through
  sequential_reader, engine.read_field/write_field, and
  materialize_struct_aligned. Previously a per-field endian override was
  silently ignored, misreading multi-byte values.
- aligned-mode materialization (M2): materialize fixed-size arrays via
  their vals[i] offset-map entries and maxLength reservations as
  zero-padded fixed-size slices (trailing NULs trimmed). Previously both
  read garbage or errored.
- dead endian param (L2): drop the ignored endian argument from
  materialize_packed/materialize_aligned; endianness is read from the
  root struct.

Verification: cargo test --release (404 pass), cargo clippy --all-targets
-- -D warnings (clean), cargo build --target wasm32-unknown-unknown
--release (clean), cargo llvm-cov --release (89.60% lines).

src/bast_validation.rs

@@ -442,7 +442,6 @@ fn is_rfc3339_timestamp(s: &str) -> bool {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::schema::Endian;
     use serde_json::json;
 
     fn doc_from<'a>(root: &'a Value, name: &'a str) -> BastDoc<'a> {
@@ -463,7 +462,7 @@ mod tests {
         doc: &BastDoc<'_>,
         buffer: &[u8],
     ) -> Result<(), AlkTypeError> {
-        let value = crate::materialize::materialize_packed(doc, buffer, Endian::Little)?;
+        let value = crate::materialize::materialize_packed(doc, buffer)?;
         validate_value(doc, &value)
     }