feat: opt-in tracing instrumentation on generated entity methods

[RAprogramm]

Closes #118

Summary

Make logger output pinpoint which entity failed without per-call boilerplate. Every generated async method (CRUD on `Repository`, transaction adapter, projection lookups, save, stream subscriber) is now wrapped with:

```rust
#[cfg_attr(
feature = "tracing",
::tracing::instrument(
skip_all,
fields(entity = "User", op = "create"),
err(Debug),
)
)]
```

Opt-in via Cargo feature → zero cost when off, identical bits to today.

What's covered

Source	Methods	Op label
`sql/postgres/crud.rs`	create / find_by_id / update / delete / list	`create` / `find_by_id` / `update` / `delete` (or `soft_delete`) / `list`
`sql/postgres/lookup.rs`	find_by_ / exists_by_	dynamic
`sql/postgres/projections.rs`	find_by_id_	dynamic
`sql/postgres/save.rs`	save (aggregate root)	`save`
`transaction.rs` (proc-macro)	tx.create / tx.find_by_id / tx.update / tx.delete / tx.list	`tx.`
`streams/subscriber.rs`	new / recv / try_recv	`stream.subscribe` / `stream.recv` / `stream.try_recv`
`entity-core::Transaction` (manual)	run / run_with_commit	`tx.run` / `tx.run_with_commit`

Helper

`crates/entity-derive-impl/src/utils/tracing.rs` — single source for the attribute emission. 5 unit tests lock the shape:

• emits `#[cfg_attr(feature = "tracing", …)]`
• carries `entity` and `op` fields
• enables `err(Debug)`
• uses `skip_all`
• references `::tracing::instrument` via absolute path

Cargo wiring

• `entity-core` — feature `tracing = ["dep:tracing"]` + optional `tracing` dep.
• `entity-derive` — feature `tracing = ["entity-core/tracing"]`. Added `tracing` as dev-dep so the test crate compiles with `--all-features`.
• All 10 example crates declare a no-op `tracing = []` feature to silence `unexpected cfg condition value: "tracing"` warnings under Rust 1.80+.

Version bump (new feature → minor)

Crate	Old	New
entity-core	0.5.4	0.6.0
entity-derive-impl	0.5.2	0.6.0
entity-derive	0.7.3	0.8.0

Breaking change in entity-core

`Transaction::run` and `run_with_commit` gain a `core::fmt::Debug` bound on the closure's error type `E`. `sqlx::Error` and all standard error types already satisfy it, so most users won't notice — but technically it is breaking. Justifies the minor bump from 0.5 to 0.6.

Usage

```toml
[dependencies]
entity-derive = { version = "0.8", features = ["tracing"] }
tracing = "0.1"
tracing-subscriber = "0.3"
```

Initialize a subscriber and you get logs like:

```
ERROR entity.User.create: error=database error: duplicate key value violates unique constraint
in entity.User.create with entity="User" op="create"
```

Test plan

• `cargo test --all-features` — 543 + 9 + 45 + 2 tests pass
• `cargo clippy --all-targets --all-features -- -D warnings` — clean
• `cargo +nightly fmt --all -- --check` — clean
• All 10 examples compile cleanly without deprecation/cfg warnings
• Codecov green before merge

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/entity-core/src/transaction.rs	0.00%	2 Missing ⚠️

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