docs: add project context and conda workaround to CLAUDE.md

- Add FASTQ domain concepts summary
- Document compression modes
- Add conda/glibc linker workaround instructions

Author: LessUp

CLAUDE.md

@@ -11,6 +11,15 @@ Read [`AGENTS.md`](AGENTS.md) first. This file only adds Claude-specific guidanc
 - Avoid `/fleet` unless the task clearly benefits from parallelism.
 - Use autopilot only after `proposal.md`, `design.md`, and `tasks.md` are in place and bounded.
 
+## Project-specific context
+
+This is a **FASTQ compression tool** for bioinformatics. Key domain concepts:
+
+- **FASTQ format**: 4-line records (ID, sequence, +, quality)
+- **Block-indexed archive**: Enables random access via block index
+- **Read length classification**: Short (ABC), Medium/Long (Zstd)
+- **Compression modes**: Archive (default), Streaming (`--streaming`), Pipeline (`--pipeline`)
+
 ## Validation
 
 ```bash
@@ -20,3 +29,17 @@ cargo test --lib --tests
 cargo doc --no-deps
 npm run docs:build
 ```
+
+## Conda/glibc workaround
+
+If tests fail with `__tunable_is_initialized@GLIBC_PRIVATE`, ensure `.cargo/config.toml` uses system GCC:
+
+```toml
+[target.x86_64-unknown-linux-gnu]
+linker = "/usr/bin/gcc"
+```
+
+Or run with:
+```bash
+PATH="/usr/bin:/bin:/usr/local/bin:$HOME/.cargo/bin" cargo test
+```