feat(pdftract-uyhq7): scaffold libpdftract cdylib+staticlib crate

Add pdftract-libpdftract as 4th workspace member with dual crate-type
configuration (cdylib + staticlib) for C/C++ SDK flexibility.

Changes:
- Create crates/pdftract-libpdftract/Cargo.toml with cdylib+staticlib
- Create crates/pdftract-libpdftract/src/lib.rs scaffold
- Update root Cargo.toml workspace.members
- Configure [lib] name="pdftract" for correct artifact naming

Artifacts produced:
- target/debug/libpdftract.so (shared, cdylib)
- target/debug/libpdftract.a (static, staticlib)

Acceptance criteria:
- PASS: cargo build -p pdftract-libpdftract produces libpdftract.so/.a
- PASS: Workspace cargo build builds all 4 crates without regression
- PASS: cargo metadata shows pdftract-libpdftract in workspace members
- PASS: nm -D shows no exported symbols (empty API scaffold)

References: pdftract-uyhq7, Phase SDK epic

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["crates/pdftract-core", "crates/pdftract-cli", "crates/pdftract-py"]
+members = ["crates/pdftract-core", "crates/pdftract-cli", "crates/pdftract-py", "crates/pdftract-libpdftract"]
 exclude = ["tests/fixtures/generate_lzw_fixtures.rs"]
 
 [workspace.package]

crates/pdftract-libpdftract/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "pdftract-libpdftract"
+version.workspace = true
+license.workspace = true
+edition = "2021"
+
+[lib]
+name = "pdftract"
+crate-type = ["cdylib", "staticlib"]
+
+[dependencies]
+pdftract-core = { path = "../pdftract-core" }
+serde_json = "1"
+libc = "0.2"
+
+[build-dependencies]
+cbindgen = "0.27"

crates/pdftract-libpdftract/src/lib.rs

@@ -0,0 +1,14 @@
+//! pdftract-libpdftract — C/C++ FFI library for pdftract.
+//!
+//! This crate provides the extern "C" API surface for C/C++ integrations.
+//! It compiles to both shared (cdylib) and static (staticlib) libraries,
+//! allowing downstream projects to link dynamically or statically.
+//!
+//! ## Output artifacts
+//!
+//! - Linux: `target/debug/libpdftract.so` (shared), `target/debug/libpdftract.a` (static)
+//! - macOS: `target/debug/libpdftract.dylib` (shared), `target/debug/libpdftract.a` (static)
+//! - Windows: `target/debug/pdftract.dll` (shared), `target/debug/pdftract.lib` (static)
+
+// Public API modules will be added here in sibling beads.
+// This scaffold provides the minimal structure for cdylib + staticlib builds.

notes/pdftract-uyhq7.md

@@ -0,0 +1,72 @@
+# pdftract-uyhq7: libpdftract cdylib + staticlib crate scaffold
+
+## Summary
+
+Created the `pdftract-libpdftract` crate as the fourth workspace member of the pdftract repo. This crate provides the FFI surface for C/C++ integrations, compiling to both shared (cdylib) and static (staticlib) libraries.
+
+## Files Created
+
+1. `crates/pdftract-libpdftract/Cargo.toml` - Crate manifest with:
+   - `crate-type = ["cdylib", "staticlib"]`
+   - `name = "pdftract"` in [lib] section (ensures libpdftract.so/.a naming)
+   - Dependencies: pdftract-core, serde_json, libc
+   - Build dependency: cbindgen 0.27
+
+2. `crates/pdftract-libpdftract/src/lib.rs` - Empty scaffold with documentation
+
+3. Updated `Cargo.toml` - Added `crates/pdftract-libpdftract` to workspace.members
+
+## Verification Results
+
+### PASS: cargo build -p pdftract-libpdftract produces expected artifacts
+
+```bash
+$ ls -la target/debug/libpdftract.a target/debug/libpdftract.so
+-rw-r--r-- 2 coding users 21848552 May 23 07:28 target/debug/libpdftract.a
+-rwxr-xr-x 2 coding users  4293504 May 23 07:28 target/debug/libpdftract.so
+```
+
+- `libpdftract.so` (4.3 MB) - shared library (cdylib)
+- `libpdftract.a` (21.8 MB) - static library (staticlib)
+- `readelf -h` confirms ELF64 shared library format
+
+### PASS: Workspace cargo build builds all 4 crates
+
+```bash
+$ cargo build --workspace
+   Compiling pdftract-core v0.1.0
+   Compiling pdftract-cli v0.1.0
+   Compiling pdftract-py v0.1.0
+   Compiling pdftract-libpdftract v0.1.0
+   Finished `dev` profile [unoptimized + debuginfo] target(s) in 22.87s
+```
+
+All existing crates build successfully; no regressions.
+
+### PASS: cargo metadata shows pdftract-libpdftract in workspace
+
+```bash
+$ cargo metadata --format-version 1 | python3 -c "import json,sys; w=json.load(sys.stdin); print(len(w['workspace_members']))"
+4
+```
+
+Workspace members: pdftract-core, pdftract-cli, pdftract-py, pdftract-libpdftract
+
+### PASS: Symbol table shows no exported symbols (expected for empty API)
+
+```bash
+$ nm -D target/debug/libpdftract.so | grep -E "^[0-9a-f]+ [TW]"
+# (no output - no exported text symbols)
+```
+
+This is expected as the acceptance criteria states: "the API is empty in this bead; sibling bead populates"
+
+## Artifact Naming Convention
+
+The crate name `pdftract-libpdftract` produces artifacts named `libpdftract.so/.a` (not `libpdftract-libpdftract`) due to the `[lib] name = "pdftract"` directive in Cargo.toml. This matches the expected naming convention for the C/C++ SDK.
+
+## Next Steps
+
+- Sibling bead will populate the extern "C" API surface
+- cbindgen will be configured to generate pdftract.h header
+- pkg-config file will be added for downstream integrations