chore(cargo): declare MSRV via rust-version = "1.91"

[YOMXXX]

Summary

`rtk` already uses `str::floor_char_boundary()` (`src/cmds/system/pipe_cmd.rs:140`), stabilized in Rust 1.91.0, but `Cargo.toml` doesn't declare the MSRV. Distribution packagers on older toolchains see a confusing `use of unstable library feature` rustc error instead of a clear cargo-level MSRV diagnostic.

Reproduction

```bash

Without this PR, on Rust 1.88.0:

$ cargo check
error[E0658]: use of unstable library feature `round_char_boundary`
--> src/cmds/system/pipe_cmd.rs:140:21
```

Fix

Add `rust-version = "1.91"` to the `[package]` table.

```toml
[package]
name = "rtk"
version = "0.34.3"
edition = "2021"
rust-version = "1.91" # ← added
```

After

```bash
$ cargo +1.88 check
error: rustc 1.88.0 is not supported by the following packages:
rtk@0.34.3 requires rustc 1.91
```

Cargo gives the right diagnostic at the `[package]`-validation step, before `rustc` ever runs.

Test plan

• `cargo +1.93 fmt --all -- --check` clean.
• `cargo +1.93 clippy --all-targets` zero warnings.
• `cargo +1.93 test --bin rtk -- --test-threads=8` → 1909 passed.
• `cargo +1.88 check` reproduces the friendly MSRV error (intended behaviour).

Notes

@DavidReque expressed interest a month ago but no PR materialized — happy to defer if they'd prefer to push their own; otherwise this lands the minimal one-line change so packagers stop hitting the unhelpful rustc message.

Fixes #1402

[YOMXXX]

Reviewer guide to reduce review time:

Expert lens: Rust packaging / MSRV.

Scope is intentionally one line: Cargo.toml now declares rust-version = "1.91" because the code already uses str::floor_char_boundary(), which requires Rust 1.91. This does not change runtime behavior or dependency resolution on supported toolchains; it only makes older toolchains fail early with Cargo's clear MSRV diagnostic instead of a confusing rustc unstable-feature error.

Suggested review checks:

• Confirm Rust 1.91 is the actual minimum implied by current source usage.
• Confirm packagers are expected to see Cargo's MSRV error rather than compile into pipe_cmd.rs and fail later.
• No code-path review needed beyond the package metadata change.

[YOMXXX]

@aeppling Could I ask for a focused maintainer pass on a small first batch?

I have several PRs open, so to avoid overloading reviewers I would like to prioritize only the narrow, mergeable, CI-clean ones first:

• chore(cargo): declare MSRV via rust-version = "1.91" #2002 — one-line MSRV declaration in Cargo.toml; no runtime behavior change.
• feat(discover): rewrite bunx prisma → rtk prisma #2005 — narrow bunx prisma rewrite-table extension; routes into the existing Prisma handler.
• fix(hooks/rewrite): restore php passthrough rewrite #1983 — restores the missing php interpreter rewrite rule with boundary tests for phpunit/adjacent tools.

The larger/security-sensitive PRs can wait. If this is not the preferred way to queue reviews, I am happy to follow whatever workflow maintainers prefer.

[pszymkowiak]

Thanks @YOMXXX — clean one-line metadata fix. The crate already requires floor_char_boundary (1.91+) in pipe_cmd.rs:135 and json_cmd.rs:109, so declaring the MSRV explicitly is the right call — it turns the cryptic E0658 rustc error into a clean cargo-level diagnostic for distribution packagers. CI green across the three OSes. Approving.