[mojo][core] Update the codebase to mojo v1.0.0b2

[forfudan]

This PR updates Decimo’s Mojo toolchain and build/test workflows to target Mojo v1.0.0b2, including the switch from .mojopkg packaging to .mojoc precompiled artifacts used across CI, scripts, and Pixi tasks.

[Copilot]

Pull request overview

Updates Decimo’s Mojo toolchain and build/test workflows to target Mojo v1.0.0b2, including the switch from .mojopkg packaging to .mojoc precompiled artifacts used across CI, scripts, and Pixi tasks.

Changes:

• Bump Pixi dependencies to mojo >=1.0.0b2,<2 and argmojo >=0.7,<0.8, updating the lockfile accordingly.
• Migrate build/test/documentation flows from mojo package / decimo.mojopkg to mojo precompile / decimo.mojoc (Pixi tasks, CI workflow, helper scripts).
• Minor documentation/docstring adjustments and a Decimal128 string-slice construction update to use Span with unsafe_from_utf8.

Reviewed changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test.sh	Switch test preflight build from .mojopkg to .mojoc via mojo precompile.
tests/test_cli.sh	Update CLI test preflight to ensure tests/decimo.mojoc exists.
tests/bigint/test_bigint_special.mojo	Docstring wording/format tweaks in BigInt special tests.
src/decimo/decimal128/decimal128.mojo	Update StringSlice construction to use Span with unsafe_from_utf8.
src/decimo/bigint/special.mojo	Add docstrings for comptime constants used in factorial/product_range logic.
src/decimo/bigdecimal/special.mojo	Add docstrings for comptime constants; clarify product_range docstring raises section.
pixi.toml	Bump dependencies and migrate tasks from mojo package to mojo precompile with .mojoc artifacts.
pixi.lock	Refresh lockfile for updated mojo/argmojo/mblack versions.
examples/run_bigfloat.sh	Update helper script to build/load tests/decimo.mojoc.
.gitignore	Ignore .mojoc artifacts in addition to .mojopkg.
.github/workflows/run_tests.yaml	Update CI to build/stage/upload .mojoc artifacts and use mojo precompile.
.github/actions/setup-decimo/action.yml	Update composite action to download/stage the .mojoc artifact.

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