refactor (RingTheory/HahnSeries/HEval): remove positive order condition from definition

[ScottCarnahan]

This PR removes the positive order condition for substitution (or Hahn evaluation) of a Hahn series into a power series. When the order is not positive, the function substitutes zero. We also add short lemmas for substitution into PowerSeries.X and PowerSeries.C.

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Is there a better way to express eq_toFun?

[github-actions]

PR summary 7deb334c5f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ heval_C
+ heval_X

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[leanprover-community-bot-assistant]

This pull request has conflicts, please merge master and resolve them.

[ScottCarnahan]

This PR has been migrated to a fork-based workflow: #25830